revised Chemistry of Lanthanoides

Inorganic Chemistry
Chemistry of Lanthanoids
Dr. Reena Jain
Dept. of Chemistry
Hindu College
Delhi -110052
(26.10.2006)
CONTENTS
Introduction
Position of lanthanoids in the periodic table
Terrestrial abundance and distribution
Extraction of lanthanoids from minerals
Electronic structure
Atomic and ionic radii : Lanthanoid contraction
Oxidation states
Separation of lanthanoid elements
Magnetic properties
Colour and spectra
Chemistry of “0” oxidation state – The metal
Chemistry of lanthanoid ions in +2 oxidation state
Chemistry of lanthanoid ions in +3 oxidation state
Chemistry of lanthanoid ions in +4 oxidation state
Complex formation
Uses of lanthanoids and their compounds
Some comparisons and contrasts
Introduction
Group 3 of the periodic table contains scandium , yttrium and lanthanum . Strictly
speaking , actinium should also be included , but in practice , it is studied separately .
There are fourteen elements that follow lanthanum and these are called lanthanides . The
lanthanides comprises of the largest occurring group in the periodic table . These
lanthanides are placed below the main body of the periodic table in the manner of a
footnote . The full - width version of the periodic table shows the position of the
lanthanides more clearly (Table 1) .
Table 1: Modern Periodic Table
These
lanthanides
are associated with a major confusion regarding their terminology.
Whether the term “Lanthanides” refers to the fifteen elements from La to Lu or the
fourteen elements from Ce to has been debated for long . “Rare Earth Elements” and
“ Rare Earth Metals” are trivial names sometimes applied to a collection of these
elements in the periodic table . Earth is an obsolete term for “oxide” . At the time of their
discovery , the oxides of these elements were believed to be scarce in abundance , as
minerals . This terminology is no longer appropriate , as these elements are no longer
rare , except promethium , with a t1/2 of 2.6 years . To avoid any confusion , now the
term “lanthanoid” rather than “lanthanide” , is used to represent these elements as the
suffix “-ide” is generally used to indicate anions . However , even now there is a
confusion regarding the position of La , i.e. , whether the group is made up of fifteen
elements , La to Lu , or fourteen elements , Ce to Lu . Lanthanides are chemically similar
to each other , to scandium as well as yttrium . Currently, the general symbol, Ln, is
used for the fourteen elements ( Ce - Lu ) and group III elements Sc , Y and La.
The story of the lanthanoids begins in 1787 when a young Swedish artillery officer,
Lieutenant Carl Axel Arrhenius , who was a keen amateur geologist , was exploring a
quarry at a small town called Ytterby , near Stockholm . He found a new , very dense
black mineral which he named ytterbite . Its chemical analysis carried out by Johan
Gadolin , a Finnish chemist in 1794 showed that the new mineral contained oxides of
iron , beryllium, silicon and a new , previously unidentified 'earth' which he named 'yttria' .
Yttria was later shown to be a mixture of the oxides of six rare earth elements . The
history of discovery and naming of the lanthanides are summarized in Table 2 .
Table 2: Discovery & origin of names of lanthanoids, including yttrium, thorium &
scandium
Year
Element
Origin of
Discoverer
Nationality Comments
name
1794
Yttrium
Yttterby mine,
Sweden
After the
asteroid Ceres
(which in turn
named after a
Greek deity )
Johan Gadolin
Finn
1803
Cerium
Baron Jons
Jakob
Berzelius and
William
Hisinger
Swedish
1828
Thorium
After Thor ,
the
Scandinavian
god of war
From Greek
lathano = to
lie hidden
(because it lay
concealed in
the earth )
Derived from
Yttterby mine,
Sweden
Derived from
Yttterby mine,
Sweden
Baron Jons
Jakob
Berzelius
Swedish
1839
Lanthanum
Carl Gustav
Mosander
Swedish
1843
Erbium
Carl Gustav
Mosander
Swedish
1878
Terbium
Carl Gustav
Mosander
Swedish
1878
Ytterbium
Derived from
Yttterby mine,
Sweden
Jean Charles
de Marignac
French
1879
Samarium
After the
mineral
samarskite ,
in turn after
the minerals
discoverer , a
Russian
mining official
V. E Samarsky
Paul E . Lecoq
de
Boisbaudran
Swedish
Also discovered
independently in
same year by
Martin Heinrich
Klaproth
(German) . The
pure element was
not isolated until
1875 .
In 1907 and
1908 , Georges
Urbain (French)
and Carl Auer
Von Welsbach
(Austrian)
independently
separated
Marignac’s
ytterbium into
two elements ,
which are now
called ytterbium
and lutetium
1879
Scandium
1879
Holmium
1879
Thulium
1880
Gadolinium
1885
Praseodymium
1885
Neodymium
1886
Dysprosium
1901
Europium
After
Scandinavia
After the
Latin word
for Stockholm,
Holmia
Lars Fredrik
Nilson
Per Teodor
Cleve
Swedish
From the
Latin Thule,
an ancient
name for
Scandinavia
In the honour
of Johan
Gadolin , a
Finnish
chemist
Per Teodor
Cleve
Swedish
Jean Charles
de Marignac
Swiss of
French origin
From Greek
prasios =
green , in
reference to
the colour of
the salts and
didymos =
twin , because
the earth
didymia was
separated into
two salts ; Pr
and Nd
From Greek
neo = new
and didymos
= twin ,
because the
earth didymia
was separated
into two salts;
Pr and Nd
From Greek
dys = bad
and prositos
= aproachable,
, dysprositos
means hard to
get because of
the difficulty
involved in its
detection and
isolation
After Europe
Carl Auer
Von Welsbach
Austrian
Carl Auer
Von Welsbach
Austrian
Paul E . Lecoq
de
Boisbaudran
French
Eugene
Demarcay
French
Swedish
Also discovered
independently by
Jacques Louis
Soret and Marc
Delafontaine
(Swiss)
Paul E . Lecoq
de Boisbaudran
independently
isolated the
element from
Mosander’s yttria
in 1886
Not isolated in
relatively pure
form until 1925
1907
Lutetium
1947
Promethium
After Lutetia ,
Latin name
for the place
where Paris
was founded
After
Prometheus ,
in greek
mythology ,
who brought
fire to
mankind in
reference to
harnessing of
the energy of
the nuclear
fission and
warning
against its
dangers
Independently
by Georges
Urban and
Carl Auer
Von Welsbach
Charles
DuBois
Coryell
Lawrence E .
Glendenin and
Jacob A .
Marinsky
French and
Austrian
American
On the basis of their separablility , the lanthanoids were conveniently divided into the
“cerium group minerals” or “light earths” (including light lanthanoid elements , from La
to Euro) and the “yttrium group minerals” or “heavy earths” (including heavy lanthanoid
elements from Gd to Lu , along with Y) . Yttrium is lighter than other “yttrium group
minerals” , but is still grouped with them , as it has a comparable ionic radius and
occurs in nature associated with the ores of heavier lanthanoids. This unit shall deal
with the general chemistry of lanthanoids, including the implementation of the conceptual
approach . The unit also provides the background essential to understand the problems of
their recovery and separation , along with their applications.
Position of Lanthanoids in the Periodic Table
The lanthanoids have atomic numbers between those of barium (Z = 56) and hafnium
(Z = 72) , and hence must be placed between these two elements . Ba is an alkaline
earth metal belonging to group 2 , below Sr . Hf is present is group 4 , below Zr , thus
leaving only one place between them , which lies exactly below Υ ( Z = 39 , group 3) .
Since all the lanthanoids resemble each other in many aspects , therefore it become
necessary to accommodate all of them together at one place . This problem is solved by
placing the first element i.e. La below Υ and the remaining elements separately in the
lower part of the periodic table (Table 1).
Terrestrial Abundance and Distribution
The lanthanoid elements are not particularly rare. Apart from the unstable 147 Pm
(half life 2.6 years) of which traces occurs in uranium ores , all the lanthanoids are
actually more abundant than iodine . Cerium is the twenty - sixth most abundant of all
elements , being half as abundant as Cl and more abundant than lead . Even Tm , the
rarest after Pm , is more abundant than iodine , and Lu is more abundant than gold . The
abundance of these elements and the number of naturally occurring isotopes vary
regularly , in accordance with Harkin′s rule (Table 3) .
Table 3: Abundance of the lanthanoides in the earth’s crust by mass and number of
natural isotopes
Atomic
number
58
59
60
61
62
63
64
65
66
67
68
69
70
71
Element
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Abundance
(ppm) in
earth’s crust
66
9.1
40
0
7.0
2.1
6.1
1.2
4.5
1.4
3.5
0.5
3.1
0.8
Relative
abundance
26
37
27
40
49
41
56
42
55
43
61
44
59
Naturally
occurring
isotopes
4
1
7
0
7
2
7
1
7
1
6
1
7
2
According to this rule , the elements with even atomic numbers are more abundant and
have more stable isotopes , than those with odd atomic numbers . The graphical
representation of their abundance is given in fig 1 .
Fig. 1: Abundance of Lanthanoids in Earth’s crust
The non - existence of promethium in nature may be explained by Mattauch′s rule . This
states that if each of the two elements with consecutive atomic numbers have an isotope
of the same atomic mass , one of the isotopes will be unstable . Since Nd (Z = 60) has
stable isotopes with mass numbers 142 , 143 , 144 , 145 , 146 , 148 , 150 and 152 and
Sm (Z = 62 ) has the isotopes with mass numbers 144 , 147 , 148 , 149 , 150 , 152 and
154 , there are not many stable mass numbers available for promethium (Z = 61) . If Pm
is to have a stable isotope , it must have a mass number outside the range 142 – 150 .
The isotopes of Pm which have been identified so far are radioactive .
Many minerals are known to contain lanthanoids (Table 4) . The symbols Ce and Υ in
the table represent the elements in cerium group and yttrium group , respectively . Out of
the list of the mentioned minerals , only two, namely, monazite and bastnasite are of
commercial importance . Monazite is sparsely distributed in various rocks but , due to its
high density and inertness , it is concentrated by weathering into sand or beaches , usually
in presence of other similar minerals such as cassiterite (SnO2) . Their rich deposits
occur in Travancore , South Africa , Brazil , Malaysia , the U.S.A and Australia . In fact ,
before 1960 , monazite was the only source of lanthanoids . However , a vast deposit of
bastnaesite was explored in the Mountain Pass , California has since then become the
most important single source of lanthanoids . Apart from the U. S. A. it is also found in
Madagascar .
Table 4: Important Minerals of Lanthanoids
Minerals
(i) Monazite Sand- Mixture of orthophosphates of Ce-earths, (Ce)PO4
(ii) Bastnaesite-cerium earth fluorocarbonate,(Ce)FCO3
(iii) Cerite-A hydrated silicate of the
composition,
(Ce)3 MII H3Si3O11(M-Ca,Fe)
(i) Gadolinite or Ytterbite- A ytteriumearth , iron and beryllium silicate,
(Fe, Be)3 (Y2) Si2 O10
(ii) Xenotime –An orthophosphate of Yearth(analogous to Monozite), (Y), PO4
(iii) Euxenite- Mixture of titanates,
niobates and tentalates of Y-earths,
(Y) (Nb, Ta) TiO6. XH2O
Composition
(1) Cerium group minerals
50-70% Ce-earths(i.e.
elements of at. no. 57 to
62 calculated as oxides)
1-4% Y-earths (i.e.
elements of at. no. 63 to
71 calculated as oxides)
5-10% ThO2
1-2% SiO2
22-30% P2O5
Traces of U
65-70% Ce-earths ,
< 1% Y-earths
Traces of thorium
51-72% Ce-earths
7.6% Y-earths
Traces of Th, U, Zr
(2)Yttrium group minerals
35-48% Y-earths
(Calculated as oxides)
2-17% Ce-earths
Upto 11.6% BeO
Traces of ThO2
54-65% Y-earths
∼ 0.1% Ce-earths
Upto 3% ThO2, upto
3.5%U3O3 2-3% ZrO2
13-35% Y-earths
(Calculated as Oxides)
2-8% Ce-earths
(Calculated as Oxides)
20-23% TiO2, 25-35%
(Nb, Ta)2O5
Location of significant
deposits
Occurs in the sand beaches of
Travancore(India)
Brazil
South Africa
U.S.A.
Sweden, California, New Maxico
Sweden
Caucausus
Sweden, Norway
USA(Texas and Colorado)
Norway
Brazil
Australia, Idaho(U.S.A.)
At present , China is estimated to have the world’s largest deposits of lanthanides
(43%) and is now the largest producer of these elements . India contributes only 3%
towards the production of lanthanoids. Promethium is not available from rare earth
ores. It occurs only in traces in uranium ores where it is formed by spontaneous fission
of 238U . It was first isolated as 147Pm by exchange methods from products of nuclear
fission reaction .
Extraction of Lanthanoids from Minerals
The distribution of the lanthanoids in the two commercially important minerals ,
monazite and bastnaesite , is quite similar . Both these minerals contain metals such as
Ce , La , Nd and Pr . However , monazite typically contains 5 – 10% ThO2 and 3%
yttrium earths , which are almost absent in bastnaesite . The complex composition makes
the chemical treatment of monazite very extensive and lengthy . Moreover , though
thorium is only weakly radioactive , it is contaminated with daughter elements such as
228
Ra which are more active and hence require careful handling during the processing of
monazite .
Processing includes
cracking the minerals , recovering the lanthanoids (along with
thorium) , removing thorium if present and separating the lanthanoids . The concentration
of the mineral usually begins with gravity separation on Wilfley table. Since these
minerals are heavy , their sand gets caught up on the riffles and the gangue material is
washed off and dried . The magnetic impurities are removed by magnetic separation.
The concentrated mineral is then subjected to chemical treatment , which is technically
known as “opening up” or “cracking” . These treatments depend on the ore being used
and the extent to which the metals are to be separated from each other . The chemical
treatment of monazite is done either by NaOH or concentrated H2SO4 solution . The
principle underlying the cracking is the difference in solubilities of Ln2 (SO4)3 . Na2 SO4 .
x H2O for the light and the heavy lanthanoids and also the low solubility of the
hydrous oxide of thorium.
(i) Cracking of monazite by conc. H2SO4 – The finely powdered and concentrated ore
is treated with 93% H2SO4 at 2000 C for several hours . The reaction is exothermic and
the resulting viscous paste is leached with cold water . Th , La and the lanthanoids
dissolve as sulphates , leaving behind the insoluble residues which mainly contain
radioactive 228Ra . The solution of sulphates of Th , La and Ln , on partial neutralization
with NH4OH , precipitates out ThO2 . The remaining solution is then treated with
Na2SO4 , to salt out La and the light lanthanoids as sulphate , leaving the heavy
lanthanoids in solution . This scheme is summarized in fig.2 (a). The solution containing
the sulphates of heavy lanthanoids can then be used to separate the individual
components by various methods , such as valency change, ion exchange and solvent
extraction , discussed later in the unit .
(ii) Cracking of monazite by NaOH - The finely powdered and concentrated ore is
treated with 65% NaOH solution at 1400C , followed by extraction with water . A slurry
of impure hydrous oxides is obtained , which is treated with boiling aq. HCl until the
pH is 3.5 . Crude ThO2 separates out leaving behind a solution of impure lanthanoid
chlorides. The solution is then treated with a solution of BaCl2 and Ln2 (SO4)3 in
stoichiometric amounts. BaSO4 precipitates out, along with radioactive 228Ra as RaSO4 .
This scheme is summarized in fig.2 (b). The remaining solution containing Ln and
lanthanoid
chlorides can be used to separate
the individual components by
special
techniques.
Symposium on Symposium on Symposium on
“ Emerging Areas In Chemistry”
Dec. 1 – 3 , 2005
Hindu College
University of Delhi
Symposium on Symposium on
Fig. 2: Cracking of Monazite / xenotime by (a)conc. H2 SO 4; (b) NaOH
Out of the two cracking methods, the conc. H2SO4 treatment is more economical than the
NaOH process. However, the latter process gives higher yields and a cleaner
separation .
(iii) Cracking of bastnaesite - The bastnaesite mineral contains very small amounts
of Th and heavy lanthanoids and requires a comparatively simpler treatment . The
concentrated ore is treated with conc. H2SO4 at 2000 C , when CO2 , HF and SiF4 are
evolved . The dried product is then treated with water to obtain a solution
containing lanthanum and the light lanthanoids , which are then separated out via
special techniques discussed later in the unit . A schematic representation of the
procedure followed is given in fig 3.
Fig.3: Cracking of Bastnaesite
Electronic Structure
The electronic structure of the members of group 3 , in the modern periodic table ,
indicates that the elements usually listed in this family are the first members of the
four d – type transition series .
Sc , Z = 21
1s2 2s2 2p6 3s2 3p6 3d1 4s2
Or , [Ar] 3d1 4s2
Y , Z = 39
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d1 5s2
Or , [Kr] 4d1 5s2
La , Z = 57
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 5d1 6s2
Or , [Xe] 5d1 6s2
Ac , Z = 89
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d106s2 6p6 6d1 7s2
Or , [Rn] 6d1 7s2
In elements succeeding scandium and yttrium , the electrons are added to the 3d and
4d levels respectively , giving the first and the second transition series . However , after
lanthanum , the energy of the 4f level falls below that of the 5d and thus the
electrons are added to the inner , well - shielded 4f orbitals before entering into the 5d
subshell . Hence, the lanthanoid series is defined by the progressive filling of the 4f
orbitals , namely , z3 , xz2, yz2 , xyz , z ( x2 - y2 ) , x ( x2 - 3y2 ) and y ( 3x2 - y2 ) . The
general set f orbitals are illustrated in fig. 4. Since there are seven such orbitals ,
each with a capacity of two electrons , a total of fourteen elements of this f - type
series may result before the 5d orbitals starts filling up again . This accounts for the
elements cerium through lutetium (Z = 58 to 71).
Fig.4: Shape of seven f orbitals
The electronic configuration is established on the basis of the emission spectra of the
element under consideration . If the spectrum is simple , containing only a few lines , its
interpretation becomes easier and the correct ground state configuration can be
established for the atom in question . However , the emission spectra for many
lanthanoids is highly complex , making the establishment of an absolutely correct
configuration extremely difficult . The difficulty arises due to the fact that the 5d and
4f orbitals have comparable energy , so that the distinction between the two is not
easy . The configuration of the lanthanoids is summarized in table 5.
Table 5: Ground state electronic configuration of Lanthanoids
Element
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Atomic
number (z)
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
Electronic configuration
Idealized
observed
[Xe]5d16s2
[Xe]5d16s2
[Xe]4f15d16s2
[Xe]4f15d16s2
[Xe]4f25d16s2
[Xe]4f3 6s2
[Xe]4f35d16s2
[Xe]4f4 6s2
4
1 2
[Xe]4f 5d 6s
[Xe]4f5 6s2
[Xe]4f55d16s2
[Xe]4f6 6s2
6
1 2
[Xe]4f 5d 6s
[Xe]4f7 6s2
[Xe]4f75d16s2
[Xe]4f75d16s2
[Xe]4f85d16s2
[Xe]4f9 6s2 or
[Xe]4f85d16s2
[Xe]4f95d16s2
[Xe]4f10 6s2
10
1 2
[Xe]4f 5d 6s
[Xe]4f11 6s2
[Xe]4f115d16s2
[Xe]4f12 6s2
12
1 2
[Xe]4f 5d 6s
[Xe]4f13 6s2
[Xe]4f135d16s2
[Xe]4f14 6s2
14
1 2
[Xe]4f 5d 6s
[Xe]4f145d16s2
It clearly indicates that the 4f orbitals are not occupied regularly , mainly in case of Ce ,
Gd and Lu . Hence , the general electronic configuration for lanthanoids is [Xe] 4f114 0 2
5d 6s , with the exception of Ce , Gd and Lu . In cerium , even the sudden
contraction and reduction in energy of the 4f orbitals immediately after La , is not yet
sufficient to avoid occupancy of the 5d orbital . Gd has a 5d1 arrangement , leaving a
half filled 4f , which leads to increased stability . Lu has a 5d1 arrangement , as the f
shell is already full .
On the basis of the similarity in the outer electronic configuration , scandium , yttrium
and actinium should be placed along with the lanthanoids . However , due to the physical
limitations of the modern periodic table , the lanthanoids and actinoids are placed
separately from the main body of the periodic table . Neither scandium nor yttrium as
well as lanthanum can be called properly as lanthanoides , since the 4f orbital does not
have any electron . Property wise , yttrium and lanthanum are better discussed with the
lanthanoids than with any other elements . On the other hand , scandium is markebly
different , even though it was first isolated from yttria sources .
Atomic and Ionic Radii : Lanthanoid Contraction
In the periodic table , the atomic as well as ionic radii normally increase on descending
down a group , due to the inclusion of extra filled shells of electrons . However , on
moving from left to right across a period , the atomic and ionic radii decrease . This is
due to the fact that the extra orbital electrons are not able to shield the extra nuclear
charge completely and hence the increase in the effective nuclear charge is responsible
for the decrease in size . The atomic and ionic radii of lanthanoids are given in Table 6.
Table 6 : Size relationship
Symbol
Atomic
number
Crystal or ionic radius, A0
Atomic
radius, A0
Ln2+
Sc
Y
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
21
39
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
1.641
1.801
1.877
1.82
1.828
1.821
1.802
2.042
1.802
1.782
1.773
1.766
1.757
1.746
1.940
1.734
1.11
1.09
0.94
0.93
Ln3+
0.68
0.88
1.061
1.034
1.013
0.995
(0.979)
0.964
0.950
0.938
0.923
0.908
0.894
0.881
0.869
0.858
0.848
Ln4+
0.92
0.90
0.84
The table clearly shows that on moving from Sc to Y to La , there is a steady increase
in the atomic as well as the ionic radii . This is due to the fact that addition of electrons
to higher energy levels overcome the increasing contractive effects resulting from the
enhanced attraction produced by larger nuclear charge. However, as we move along the
lanthanoid series , there is a decrease in atomic as well as ionic radii . A similar
but more limited trend characterizes the non - tripositive ions. The contraction in size
from one element to another is fairly small , so that the additive effect over the
lanthanoid elements from Ce to Lu is just 0.2 A0 . This limited contraction in the
atomic and ionic radii of the lanthanides is known as Lanthanoid Contraction.
The shielding effect of the electrons decreases in the order s > p > d > f . In lanthanoids the
additional electron enters 4f sub - shell and not the valence shell , namely , sixth
shell. The “Lanthanoid Contraction” , therefore , occurs because , although each
increase in nuclear charge is balanced by a simultaneous increase in electronic charge ,
the directional characteristics of the 4f orbitals cause the 4fn electrons to shield
themselves and other electrons from the nuclear charge only imperfectly . Thus each unit
increase in nuclear charge produces a net increase in attraction for the whole
extranuclear electron charge cloud and each ion shrinks slightly in comparison with its
predecessor . On the other hand , although a similar overall reduction is seen in the
atomic radii , the trend for Eu and Yb is spectacularly irregular . Mathematically ,
lanthanoid contraction could be understood in terms of effective electron potential , Veff
, which is expressed as :
− Z eff
l (l + 1)
Veff =
+
r
2r 2
Or,
Veff
=
columbic potential
+
centrifugal potential
The angular momentum for an f orbital (l = 3) is large and hence the centrifugal
potential , which tends to keep the electrons away from the nucleus , is also large .
Increase in the atomic number increases the columbic attraction to a large extent for a
smaller value of n , due to a proportionately greater change in Zeff . This can be
viewed empirically , to differing penetration effects , 4f orbitals (and the atoms in
general) steadily contract across the lanthanoid series . Fig. 5 shows that the graph has
two peaks , one at Eu and the other at Yb , which possess exceptionally high values for
their atomic radii. This is regarded to be due to the difference in their metallic bonding .
Most of these metals are composed of a lattice of Ln3+ ions with a 4fn configuration
and three electrons in the 5d / 6s conduction band . Metallic Eu and Yb , however , are
composed predominantly of larger Ln2+ ions with 4fn+1 configurations and only two
electrons in the conduction band , thus leaving behind half - filled and completely filled
4f orbitals , respectively . The rough parallel between these metals and barium supports
this contention. Correspondingly , the slightly reduced atomic radius of cerium is due to
the presence of ions in an oxidation state somewhat above +3 . Similar discontinuities
are found in other properties of the metals , particularly at Eu and Yb . The decrease
in the crystal or ionic radii is indicated in Fig 6 . The general decrease in crystal radius
from Ln2+ to Ln3+ to Ln4+ reflects the effect of increasing cationic charge.
Fig.5: Atomic radii of lanthanoides, barium and Hafnium
Fig.6: Crystal radii of Ln2+ , Ln3+ and Ln4+ ions
It is to be noted that a contraction resulting from the filling of the 4f electron shell is
of course not exceptional . Similar contraction occurs in each row of the periodic table.
In the d – block for instance , the ionic radii decrease by 20.5 pm from Sc3+ to Cu3+ ,
and by 15 pm from Y3+ to Ag3+ . However, the importance of the lanthanide contraction
arises from its consequences:1] Basic character of oxides , Ln2O3 and hydroxides , Ln(OH)3 : The atomic and ionic
radii affect those properties of the metal and their respective cations , which mainly
reflect the attraction or lack of attraction for the electrons . This includes basicity . In
broad terms , basicity is a measure of the ease with which a species loses electrons
and it decreases with decrease in the atomic or ionic radii . In terms of ionic radii ,
basicity may be expected to decrease in the order :
La3+ > Ce3+ > Pr3+ > Nd3+ > Pm3+ > Sm3+ > Eu3+ > Gd3+ >Tb3+ > Dy3+ > Ho3+ > Y3+ >Er3+ >Tm3+
> Yb3+ >Cu3+ > Sc3+
Amongst the tripositive ions , due to the decrease in size from La3+ to Lu3+, the
covalent character in Ln(III) hydroxides increases . This is in accordance with
Fajan’s rule . Hence , the basicity of the hydroxides decreases from La to Lu . Thus
La(OH)3 is the most basic while Lu(OH)3 is the least basic hydroxide . The hydroxide
of more highly charged Ce4+ is even more basic than any tripositive ion . Basicity
differences are reflected in the hydrolysis of ions – more basic ion hydrolyses less
readily ; solubilities of salts ; thermal decomposition of oxy - salts – more basic oxy - salts
decompose with difficulty ; and the formation of complex species .
2] Occurrence of yttrium with heavy lanthanoids :The magnitude of lanthanoid
contraction is such that the ionic radii of Y3+ (0.88 A0) is reached in the holmium erbium region ( 0.894 - 0.881A0 ) . This similarity in ionic radii , along with the equality
in ionic charge (i.e. +3) accounts for the invariable natural occurrence of yttrium with
these heavier lanthanoids . The marked similarity in the crystal structure , chemical
properties and solubility between yttrium compounds and those of the heavier
lanthanoids make it difficult to separate yttrium from heavy lanthanoids . In fact , the
behaviour of yttrium is considered to be so characteristic of these elements that these
elements are referred to as yttrium earths . It is to be noted that the marked differences
observed between the chemistry of scandium and that of the lanthanoids are consistent
with the fact that Sc3+ is much smaller than even the Lu3+ ion .
3] Melting and boiling point : The hardness and melting and boiling points of the
elements increase from Ce to Lu . This is because of the increase in attraction
between the atoms , with decrease in size .
4] Separation of lanthanoids : The chemical properties of an ion are mainly dependent
on its size and charge . For the various Ln3+ ions , since the charge remains the same
and the decrease in size is just marginal , their chemical properties are very similar ,
making their separation difficult . But , it is this small variation in properties that
permits the separation of the lanthanoids by fractional means .
5] Effect on the post - lanthanoid elements : The elements which follow lanthanoids
in the third transition series of the periodic table , are called post - lanthanoid elements .
Due to the lanthanoid contraction , the post - lanthanoid elements are considerably
smaller than expected .
Normally , the covalent radii increase with increase in the atomic number , on moving
down the group . This , in fact , holds good when we compare the covalent radii
values of the elements of the first , second and the third transition series (Table 7 ) .
Table 7 : Covalent radii of the transition elements (Αο)
Sc
1.61
Y
1.80
La
1.87
Ti
1.32
Zr
1.45
Hf
1.44
V
1.22
Nb
1.34
Ta
1.34
Cr
1.17
Mo
1.29
W
1.30
Mn
1.17
Tc
Re
1.28
Fe
1.17
Ru
1.24
Os
1.26
Co
1.16
Rh
1.25
Ir
1.26
Ni
1.15
Pd
1.28
Pt
1.29
On comparing the covalent radii values of the elements of the second transition series
with those of the third series , it is observed that the normal increase in the covalent
radii values from Y to La disappears after lanthanoids . Thus pairs of elements such
as Zr / Hf , Nb / Ta and Mo / W are almost identical in size . The close similarity of
properties in such a pair makes their chemical separation difficult . The sizes of the
third row of the transition elements are very similar to those of the second row of
transition elements . Thus , the second and the third rows of transition elements
resemble each other more closely than do the first and second rows . These similarities
between members of the second and third transition series continue through the
platinum metals to at least silver and gold . As a consequence of the contraction in the
atomic radii of the elements of the third transition series , the packing of atoms in their
metallic crystals become so much compact that their densities become very high . Thus ,
while the densities of the elements of second transition series are only slightly higher
than those of the elements of first series , the densities of the elements of third
transition series are almost double than those of the elements of the second transition
series . Hence , the increase in atomic mass from 91.2 to 178.4 g/mol and almost
similar radii leads to an increase in density from 6.51 to 13.35 g/cm3 , on moving
from Zr to Hf . Indeed , due to similar chemical behaviour , Hf (1923) was
discovered 134 years later than Zr(1789) .
Oxidation States
The ionization enthalpy and standard electrode potential values for
the lanthanoids
are listed in table 8 . The sum of the first three ionization enthalpy values is low.
Thus , the oxidation state ( + 3 ) is ionic and the stability of Ln3+ dominates the
chemistry of these elements . In just the same way as for other elements , the higher
oxidation states occur in the fluorides and oxides and the lower oxidation states occur
in the other halides , mainly bromides and iodides . Oxidation numbers (+2 ) and (+4)
do occur , particularly when they lead to a noble gas configuration (f0 in Ce4+) , or a
half - filled f shell (f7 in Eu2+ and Tb4+) , or a completely filled f shell (f14 in Yb2+) . In
addition , (+2 ) and (+4) states also exist for elements that are close to these
configurations . Thus , Sm2+ and Tm2+ occur with f6 and f13 configurations , while Pr4+
and Nd4+ have f1 and f2 configurations , respectively (table 9 ) . However , of the non
- tripositive state , only tetrapositive cerium , praseodymium , and terbium and dipositive
samarium , europium and ytterbium have sufficient chemical stability .
Table 8 : Some properties of the lanthanoid elements
Element Ionization
enthalpy
/ kJmol-1
1st
2nd
Ce
541 1047
Pr
522 1018
Nd
530 1034
Pm
536 1052
Sm
542 1068
Eu
547 1085
Gd
595 1172
Tb
569 1112
Dy
567 1126
Ho
574 1139
Er
581 1151
Tm
589 1163
Yb
603 1175
Lu
513 1341
E0(M4+/M3+) E0(M3+/M2+) E0(M3+/M) ∆atmH
∆hydH
/V
/V
/V
/kJmol- /kJmol3rd
1940
2090
2128
2140
2285
2425
1999
2122
2230
2221
2207
2305
2408
2054
1.61
∼2.860
2.7
-
-1.000
-0.360
-1.205
-
-2.483
-2.462
-2.431
-2.423
-2.414
-2.407
-2.397
-2.391
-2.353
-2.319
-2.296
-2.278
-2.267
-2.255
1
1
419
356
328
301
207
178
398
389
291
301
317
232
152
-
-3370
-3413
-3442
-3478
-3515
-3547
-3571
-3605
-3637
-3667
-3691
-3717
-3739
-3760
Table 9 : Distinguishing electronic configuration for observed oxidation states
Symbol
Configuration
+2
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
[Xe]4f2(CeCl2)
[Xe]4f4(NdI2)
[Xe]4f6(Sm2+)
[Xe]4f7(Eu2+)
[Xe]4f13(TmI2)
[Xe]4f14 (Yb2+)
+3
[Xe]4f0(La3+)
[Xe]4f1(Ce3+)
[Xe]4f2(Pr3+)
[Xe]4f3(Nd3+)
[Xe]4f4(Pm3+)
[Xe]4f5(Sm3+)
[Xe]4f6(Eu3+)
[Xe]4f7(Gd3+)
[Xe]4f8(Tb3+)
[Xe]4f9(Dy3+)
[Xe]4f10(Ho3+)
[Xe]4f11(Er3+)
[Xe]4f12(Tm3+)
[Xe]4f13(Yb3+)
[Xe]4f14(Lu3+)
+4
[Xe]4f0(Ce4+)
[Xe]4f1(PrO2 ,
Na2PrF6)
[Xe]4f2(Cs3NdF7)
[Xe]4f7(TbO2 ,
TbF4)
[Xe]4f8(Cs3DyF7)
The direct correlation between oxidation state and electronic configuration is the
exception rather than the rule . Since , 6s2 configuration is the most characteristic
configuration for the lanthanoids, a uniform dipositive state might be expected . However,
the dominance of the +3 oxidation state results from the stabilizing effects of
increasing positive charge on different orbitals . When electrons are removed from a
lanthanoid atom , the orbitals are stabilized in the order 4f > 5d > 6s . This is the same
order in which the orbitals penetrate through the inert core of electrons towards the
nucleus . By the time an ionic charge of +3 is reached , the 4f have become core like , with any remaining electrons occupying the 4f , leaving 6s and 5d orbitals
empty . Removal of further electrons from the core then becomes energetically
unfavourable. This explanation can further be expanded by examining the relative
stabilities of other oxidation states from a thermodynamic view point with the help
of fig. 7 , 8 , and 9 .
Fig.7: Plot of I1 , I2 , I3 , I4 and ∆Hatom for lanthanoids
Fig. 8(a): Plot of [ I1 + I2 + I3 + ∆atomH]
Fig. 8(c): Plot of (c) [I1 + I2 + I3 +∆atomH + ∆hydH(Ln3+)]
Fig. 8(b): Plot of (b) [ ∆hydH(Ln3+)]
Fig. 9 : Plot of [ I1 + I2 + I3 + I4 + ∆atomH]
Stability of +2 oxidation state - a so called “ anomalous” state
The stability of the +2 oxidation state can be studied by examining ∆rG for a
process such as :
Ln2+ (aq) + H + (aq)
Ln3+ (aq) + ½ H2 (g)
A thermodynamic cycle for this reaction can be constructed as:Ln2+ (aq) +
H+ (aq)
∆rH
+∆hydH(Ln2+)
Ln2+(g)
+
Ln3+(aq) + 1/2 H2(g)
-∆hydH(Ln3+)
H+(aq)
I3 + (∆hydH(H+))
Ln3+(g) + 1/2 H2(g)
According to the Hess’s law of constant heat summation :
∆rH = ∆hydH(Ln2+) + I3 + ∆hydH(H+) + (-∆hyd H(Ln3+))
also ,
∆rG = ∆rH -
T∆rS
Hence , ∆rG = [ ∆hydH(Ln2+) + I3 + ∆hydH(H+) + (-∆hyd H(Ln3+) )]
-
T∆rS
∆hydH(Ln2+) , ∆hydH(H+) and (-∆hyd H(Ln3+)) represent the enthalpy of hydration for Ln2+ ,
H+ and Ln3+ , respectively . I3 is the third ionization enthalpy for the lanthanoid ,. ∆rH
is the enthalpy of reaction , T is the temperature , ∆rG is the Gibb’s free energy and
∆rS is the entropy change for the reaction . Here , ∆hydH(H+) remains constant and can
be ignored , as variation in ∆rG across the Ln series is sought . Due to the lack of data ,
it is assumed that ∆rS is constant across the series – as for the 3d metals . Hence,
∆rG = I3 + [ ∆hyd H(Ln2+) + (-∆hyd H(Ln3+) )] + constant
Since the ionic radii moves smoothly along the series, ∆hydH(Ln2+) and ∆hydH(Ln3+)
are also expected to vary regularly . ∆hydH(Ln3+) is larger in magnitude than
∆hydH(Ln2+) due to the higher charge and smaller radius of the ion with higher
oxidation state . The term in square brackets is thus negative and favours the +3
oxidation state. As the ions gets smaller across the series , this term smoothly
increases so that the +3 state becomes more favoured . On the contrary , I3
increases across the series , disfavouring the existence of +3 oxidation state . However
, the dominance of the +3 oxidation state arises from the balance between the increasing
enthalpy of hydration and the increasing ionization enthalpy . The occurrence of the +2
state for some of the lanthanoids arises from the non - smooth changes in the third
ionization enthalpy values .
The overall trend across the series, in general, is for the increase in I3 values. This
is due to the increase in nuclear charge across the f – block. This outweighs the increasing
repulsion between the electrons. However , the ‘break’ between f7(Eu2+ ) and f8(Gd2+)
cannot be explained easily . This break is actually due to the extra stability associated with
the f7 (half – filled) configuration. Similarly the higher than expected I3 for Yb2+ with
f14 (fully - filled) configuration can also be explained on the same basis . This can be
explained even on the basis of the Hund’s rule , according to which the electrons
prefer to adopt arrangements in degenerate orbitals so as to have maximum number of
parallel spins . Destroying pairs of such parallel spins requires extra energy. In
Gd2+(f8) , there are seven electrons with parallel spin and one with anti - parallel spin .
A pair of electrons with anti - parallel spins repels more than ‘expected ’ and so is
destabilized , leading to a low I3 , despite the increase in nuclear charge . A similar
decrease in I3 is observed on moving from Yb2+ to Lu2+. To understand small
irregularities in the third ionization energy values, a detailed inspection of the
electronic configuration is required. For example , for Pr2+ (4f3 ) and Pm2+(4f5 ) , Hund’s
rule predicts the lowest energy by way of arranging the electrons as :
ml
3
2
1
0
-1
-2
-3
All the three electrons have ml values of the same sign and can thus be taken to
be orbiting in the same direction . There is slightly less repulsion between electrons
which are orbiting in the same direction as they come across one another less
frequently . The electrons are thus repelling one another slightly less than expected and
hence ionization enthalpy is higher than expected. Now consider Pm2+(4f5) , for which
the lowest energy arrangement is :
ml
3
2
1
0
-1
-2
-3
The electron that is removed upon ionization is that with ml = -1 . As this is
orbiting in the opposite direction to the other electrons, it is repelled slightly more
than ‘expected’ and its ionization becomes easier. Thus, the ionization enthalpy of
Pm2 + is less than expected. This is known as quarter - shell effect. An identical
situation occurs between f10 and f12 in the second half of the series leading to
the three quarter - shell effect. The I3 values of Nd and Dy are thus larger than
expected, leading to the occurrence of their compounds in the +2 oxidation state.
Amongst the various
lanthanoids
in dipositive state, samarium, europium, and
ytterbium have sufficient chemical stability to be of importance to exist in aqueous
solution and to form series of compounds. Standard electrode potential values
indicate that in aqueous solution, these ions are all strong reducing agents, with
reducing strength decreasing as :
Sm2+ > > Yb2+ > > Eu2+
Stability of +3 oxidation state – the “characteristic” state
The observed oxidation states of the lanthanoids noted either in solution or in
insoluble compounds (Table 9) , clearly indicates that whatever is the electronic
configuration of the lanthanoids in the ground state , they all form the tri - positive
ions . This can be examined more closely by considering the following thermodynamic
cycle :
Ln (s)
∆H0
- ∆hyd H(Ln3+)
∆atmH
Ln (g)
Ln3+ (aq)
I1 + I2 + I3
Ln3+ (g)
According to the Hess’ s law of constant heat summation :
∆H0 = ∆atmH + I 1 + I2 + I3 + (-∆hyd H(Ln3+))
where ∆atomH is the enthalpy of atomization of the lanthanide ; I1, I2 and I3 are
the first, second and third ionization enthalpies respectively and ∆hydH(Ln3+) is the
enthalpy of hydration for Ln3+(g) . The standard electrode potential, E0(Ln3+ (aq) / Ln (s) ) can
be calculated using a thermodynamic relation
∆rG0 = ∆H0 - T∆rS0 = - n FE0
The value of standard electrode potential of a lanthanoid thus depends on the
balance between the energy supplied in the form of enthalpy of atomization and the
sum of the first three ionization enthalpies [ ∆atm H + I1 + I2 + I3 ] , and the energy
released in the form of enthalpy of hydration for Ln3+(g) . These enthalpy terms are
graphed in Fig 8. The production of Ln3+ (g) shows a smooth trend, based on size
effects. Shell structure effects superimposed with clear maxima at half shell (f 7 ) and
full shell (f 14) are also reflected . The enthalpy of hydration of Ln3+(g) shows only
a smooth ionic size based trend and no shell structure effects . Balance of trends in
ionization enthalpy along with enthalpy of atomization and hydration reflects the
dominance of the shell effects over the size effects.
It can also be shown that the tri-positive state is a more preferred state in
comparison to +2 and +4 state. In other words, in aqueous solution, oxidation
represented by the equation:
1
1
Ln3+(aq) +
O2 (g) + H+ (aq)
Ln4+ (aq) +
H2O (l)
4
2
or reduction represented by the equation :
1
Ln3+ (aq) +
H2 (g)
Ln2+ (aq) + H+ (aq)
2
are not very favourable . The conversion from one oxidation state to another in
aqueous solution is controlled by the magnitudes of the energy required to remove
an electron from the gaseous ion in its lower oxidation state (i.e. , ionization
enthalpy) and of the energy released when gaseous ions combine with water to form
the hydrated species (enthalpy of hydration) . Calculations show that in solution , all
tetra - positive species (except possibly Ce4+) and all di - positive species (except Eu2+)
must revert to the tri – positive state . This leads to an important conclusion that
the tri - positive state owes its general stability to a somewhat fortuitous combination
of ionization and hydration enthalpies rather than to any particular electronic
configuration .
The ease of formation of the tri - positive state in solution is indicated by the
standard electrode potential data , summarized in table 8 . It is apparent that the
elemental lanthanoids are very powerful reducing agents and their oxidation to the tri –
positive state occurs readily and vigorously.
Similarly , oxidation and reduction among solid compounds can be related to the
magnitude of the ionization enthalpy and the energy released when the gaseous ions
combine to produce crystalline solids (i.e., the enthalpy of lattice formation) . Again ,
a fortuitous combination of these energy terms renders the tri - positive state the
most common , but the energy conditions are more favourable to the existence of
non tri - positive species (table 9) in the solid state than in solution .
Stability of +4 oxidation state - Another “Anomalous” state :The occurrence of the +4 oxidation state for Ce , Pr and Tb can be treated in a
similar manner by examining ∆rG for a process such as :1
LnX3 (s) +
X 2(g)
LnX4 (s)
2
where X is a halide .
A thermodynamic cycle for this reaction can be constructed as :
1
LnX3 (s) +
X2(g)
∆rH
LnX4(s)
2
∆diss H
U (LnX3)
X (g)
-U (LnX4)
-∆egH
Ln3+(g) +
3X-(g) +
X- (g)
I4
Ln4+(g)
+ 4X- (g)
where , U is the enthalpy of lattice formation , ∆diss H is the enthalpy of dissociation
of X2 and ∆eg H is the electron gain enthalpy for X(g) . According to Hess’s Law
of constant heat summation :
∆rH = U(LnX3) + ∆dissH (X2) + (- ∆egH (X)) + I4 +
(-U (LnX4))
Or , ∆rG = [ I4 + [ U (LnX3) + ( - U(LnX4))] +
Or, ∆rG
= I4 + [ U (LnX3) – U(LnX4) ] +
∆diss H(X2)
+ (- ∆egH(X))] -
T∆rS
constant
In an ionic model , the enthalpy of lattice formation is directly proportional to
the charges of the ions (Z+ and Z) and inversely proportional to the sum of the
ionic radii ( r+ and r-) i.e.
(Z + Z − )
U ∝
(r+ + r− )
Since the charge Z+ on lanthanoid is higher in LnX4 and the radii of Ln4+ is
smaller than that of Ln3+ , the enthalpy of lattice formation is greater for LnX4 .
Hence , the terms in the square brackets become negative , to favour
the +4
oxidation state . As the radii of the lanthanoide ion in +3 and +4 oxidation
states are expected to decrease smoothly across the series , this term also gets
more negative across the series . However , the high values of I4 favours the
existence of lanthanoide ions in +3 oxidation state , but still the +4 oxidation state
is stable where the fourth ionization enthalpy values are low ( fig. 7 ) . Ce(+4) and
Pr (+4) can be formed as the nuclear charge is lowest at the start of the series .
Tb(+4) can be formed as I4 corresponds to the process of removal of an electron
from Tb(+3) with f8 configuration to form Tb(+4) , with a more stable f7 configuration .
The plot of (∆atmH + I1 + I2 + I3 + I4) also favours +4 oxidation for Ce , Pr and Tb ,
to some extent (fig . 9) .
Separation of the Lanthanoid Elements
As discussed earlier, the lanthanoids are all typical trivalent , almost identical in
size and chemical properties . Hence, the separation of one lanthanoid from another
is an exceedingly difficult task . The methods of separation exploit slight differences
in their basic properties , stability and solubility . Most of the methods involved in
separation are fractional in character , where in each step there is a concentration of
one species at the expense of the other , but the separation results only if that step
is repeated many times . The efficiency of a fractional procedure is given by the
magnitude of its separation factor (∝). For two lanthanoids , Ln and Ln′ , being
changed from an initial concentration condition (1) to a final one (2) , the
separation factor , ∝ may be defined as
C Ln ( 2 ) / C Ln ' ( 2 ) C Ln ( 2 ) .C Ln '(1)
∝ =
=
C Ln (1) / C Ln '(1) C Ln '( 2) .C Ln (1)
No separation results for ∝ = 1 , but greater the departure of ∝ from unity , the
more efficient the process is . Some classical techniques for the separation of
lanthanoides are discussed below :
Fractional crystallization: This technique utilises the slight differences in the
solubility of various lanthanoid salts. Various anions found to form soluble salts
with tri – positive lanthanoid ions have been investigated . However , only those anions
which yield easily crystallizable isomorphous salts , possessing measurable solubility
differences as the Ln3+ ion changes , can be effective . Such systems are very limited .
The basicity decreases along the series , resulting in decrease in solubility of their
hydroxides , oxides , carbonates and oxalates . Thus, the salts at the Lu end of the
series crystallize out first . Not only simple salts such as Ln (BrO3)3. 9H2O and Ln
(NO3)3. 5H2O , but even double salts like Ln(NO3)3 . 3Mg(NO3)2. 24H2O and K2CO3
. Ln 2 (CO3)2 .12H2O are used .
This technique is very tedious and needs to be repeated many times to obtain
good separation. The mixed solution of lanthanoid salts in water is evaporated to
such an extent that about half of the dissolved salts crystallize out on cooling. The
crystals obtained are separated , dissolved and the solution is again evaporated , to
such an extent that on cooling it would deposit about one – half of the dissolved salts
as crystals . Even the mother liquor is further evaporated, so that about one – half of
the dissolved salt is obtained as crystals. Each series of operation gives one more
fraction and such series of operation are repeated a number of times, until the
desired degree of separation is achieved . The systematic and tedious fractional
crystallization of any given mixture of lanthanoids results in the separation of less
soluble components (such as the salts La3+ , obtained as crystals) from the more
soluble components (such as the salts of Sm3+ obtained in the mother liquor) . The
intermediate components such as the salts of Pr3+ and Nd3+ , are distributed between
the final crop of crystals and the mother liquor , in the order of solubility .
Fractional crystallization is normally most effective at the lanthanum end of the series
where the solubilities differ the most . The most effective compounds used are :Double ammonium nitrates , Ln (NO3)3. 2NH4NO3 . 4H2O for the removal of
(1)
lanthanum and the separation of praseodymium from neodymium.
The double manganese nitrates , 2Ln (NO3)3. 3Mn(NO3)2. 24H2O for the
(2)
separation of members of cerium group (but not the yttrium group) .
(3) The bromates , Ln (BrO3)3. 9H2O , and the ethyl sulphates , Ln (C2H5SO4)3 .
9H2O , for the separation of the members of the yttrium group .
Separation in the samarium - europium - gadolinium region and within the yttrium
group , using this technique is inefficient and often involves hundreds or even
thousands of crystallizations . Even a non - lanthanoid such as Bi (III) can be used
as a separating ion . If bismuth (III) nitrate is added to a mixture of cerium
group nitrates , containing magnesium ion , its double magnesium nitrate crystallizes
between the isomorphous ones of Sm3+ - Bi3+ and Bi3+ - Eu3+ mixtures . Bismuth
can be removed readily with hydrogen sulphide .
This tedious method of separation is later on, however, replaced by a more
effective technique i.e. solvent extraction technique, which can be used on a large scale.
Fractional Precipitation Method :
The separation by this method depends
fundamentally upon the differences in the basic character of hydroxides of
lanthanoid elements . However , these differences can be operative only when there
exists true equilibration between the solution and the solid phase . The condition is
hardly fulfilled in practice and for addition of a precipitating anion to a solution
of mixed lanthanoids , all the species are precipitated out due to localized excess of
reagent . The precipitates are then separated out and are dissolved . The redissolution
may then be too slow or too far from completion to permit true equilibration before
more reagent is added . This problem is avoided either by diluting the gaseous
precipitating agent such as ammonia with inert nitrogen gas or by generating the
precipitating ion slowly and homogeneously throughout the solution. Examples of
such precipitating agents include generation of hydroxyl ion by the cathodic
decomposition of water , carbonate and hydroxyl ions by adding urea and heating , and
oxalate ion by thermally hydrolyzing dimethyl oxalate . Another method of
controlling the precipitating agent is by addition of another reagents that may give a
complex species . For example , addition of solid oxides (eg. MgO, Ln2O3) or
carbonates (eg. PbCO3) provides a slowly increasing and controlled source of
hydroxyl ions . Sometimes , even a weak complexing agent like EDTA is added before
the addition of a precipitant . These reagents tie up all of the lanthanoid ions and
hence the least readily complexed lanthanoid ion is the most available one for
precipitation .
Let us now discuss this technique in detail by using hydroxyl ions as precipitants .
The basic character of the hydroxides of lanthanoid elements decreases from La to Lu.
Hence , the hydroxides of “cerium group minerals” are more basic than those of
“yttrium group minerals”. On adding a precipitating agent , like hydroxyl ions into a
solution containing a mixture of lanthanoid nitrates , the hydroxides of the “heavy
earths” are preferentially precipitated out , with those of “light earths” being left
behind , in the solution . Within a given sub - group, the individual lanthanoids are
separated by further fractional precipitation. However, the complete separation of
the adjacent members in a given sub – group by this method is not possible, since
the basicity differences in the hydroxides of lanthanoid elements are small.
Fractional Thermal Decomposition Of Oxy Salts: The temperature at which an
oxy salt (eg. acetate , sulphate , nitrate) is converted into an oxide or a basic salt of
lower solubility , decreases with decreasing basicity of the tri – positive cation .
Yttrium is concentrated into the more basic fractions and hence is effectively
separated from the heavier lanthanoids .
When a mixture of Ln (NO3)3 is fused, it undergoes thermal decomposition, i.e. a
temperature will be reached when the least basic nitrate changes to the oxide . The
mixture is leached with water , the nitrates are dissolved and can be filtered off ,
leaving behind the insoluble oxides . The oxides are then removed, dissolved in
HNO3 and the process is repeated.
Apart from these classical methods of separation which are generally employed for
small scale separation, some modern techniques have also been devised. Some of the
modern methods now used for separating individual lanthanides are discussed below:
Solvent Extaction : The separation of lanthanoids by this method was introduced
to separate and identify various lanthanoids produced by the fission of uranium.
This method is important and used for large scale separation. This
based on the difference in solubility of lanthanoid salts in water
immiscible organic solvent. For example, the heavier lanthanoid ions
soluble in an organic solvent, than the lighter ions. Their solubilities in
ionic solvents, however, are reversed.
method is
and in an
are more
water and
The technique involves the extraction of aqueous solution of lanthanoid salts in a
continuous counter - current process into a non - polar organic liquid. The most widely
used extracting solvent is tri - n - butyl phosphate , (m BuO)3 PO , ( TBP ) , in an inert
medium like kerosene , to extract the lanthanides from nitric acid solution . TBP form
complexes with Ln3+ (aq) ions in presence of NO3- ion and hence, the complex
moves into the organic phase.
Ln3+ (aq) + 3NO3- (aq) + 3TBP(org)
Ln (NO3)3 (TBP)3 (org)
The distribution ratio (λ) represents the distribution of lanthanoid ions between the
two phases .
λ = Total concentration of solute in one solvent
Hence ,
Total concentration of solute in other solvent
=
[Ln(NO3)3(TBP)3(org)]
[Ln3+(aq)]
For two tri – positive lanthanoide ions , Ln/ 3+ and
∝ is given as ,
λ'
∝ =
Ln3+, the
separation factor ,
λ
=
[Ln/ (NO3)3(TBP)3(org)][Ln3+(aq)]
[Ln (NO3)3(TBP)3(org)][Ln/ 3+(aq)]
The distribution equilibrium may be described as :
K
=
[Ln (NO3)3(TBP)3(org)]
[Ln3+(aq)][NO3-(aq)]3[TBP(org)]3
Hence , a relation between K and λ can be established as :
λ
=
K [NO3- (aq)]3 [ TBP (org)]3
A plot of log10 λ vs. [NO3- (aq)] at constant concentration of TBP should give
a straight line with a slope 3 . This is , however , true for cases with the
concentration of nitric acid above 10 M . It is also to be noted that the extent
of extraction increases with decreasing atomic radii .
A mixture of La (NO3)3 and Gd (NO3)3 can be easily extracted using TBP. The
separation factor for La(NO3) and Gd(NO3)3 is 1.06 , for 15.8 M nitric acid – 100%
TBP system . The difference no doubt is very small, but by using this technique
a very large number of partitions can be performed automatically.
Another organic solvent that can be used as extractant is bis(2-ethyl hexyl)
phosphoric acid i.e. DEHPA ( fig 10(a)). Lanthanoids of high purity (∼ 95%) have been
obtained using this solvent. This reagent is a more efficient extractant , as well as
more resistant to hydrolytic decomposition (in presence of aqueous nitric acid ) in
comparison to TBP. Esters of this type better represented as HDGP exists as
dimers in the organic phase.
Ln3+ (aq) +
Ln[(HDGP)2]3 (org) + 3H+ (aq)
3(HDGP)2 (org)
The distribution ratio may be derived to be
λ
=
K [(HDGP)2 (org)]
[H+ (aq)]3
An average separation factor of 2.5 for adjacent lanthanoids has been reported .
Such esters form chelates , having the units of the type drawn in fig 10(b).
CH3(CH2)3 CH(Et) CH2O
OH
P
CH3(CH2)3 CH(Et) CH2O
O
(a)
CH3(CH2)3 CH(Et) CH2O
O
H
O
P
CH3(CH2)3 CH(Et) CH2O
OCH2
P
O
O
OCH2
Ln/3
(b)
Fig 10 : Structure of (a) DEHPA ; (b) a unit of [Ln(HDGP)2]3
Solvents such as mono alkyl phosphoric acids , RP(O) (OH)2 ; pyro phosphate esters ,
(RO)2P(O)-O-P(O)(OR)2 ; 1 , 3 - diketones and 8 – quinolinols have also been used for
extraction . However , these solvents are not as successful as TBP and DEHPA .
Solvent extraction , no doubt is an efficient technique for separation of lanthanoids .
This technique is mainly helpful in separation and purification of Ln3+ ions from
Ce4+ and Th4+ ions , using TBP . La3+ is somewhat less readily extracted than the
other tripositive ions and hence can be separated out easily . This technique is
also useful for separation and purification of scandium .
Complex Formation : - In the presence of complexing agent like ethylenediammine
tetraacetic acid ie : EDTA , the lanthanoids can be separated by precipitation , as
oxalates .
When a mixture of lanthanoids is treated with EDTA , all the ions form
complexes . However , they differ in their stabilities . The heavy lanthanoids form
stronger complexes than the lighter lanthanoids . The oxalates of the lanthanoids are
insoluble but addition of oxalate ions to this solution does not give a precipitate ,
since the La3+ ions are not freely available and are complexed with EDTA . If some
acid is also added to the solution , the less stable EDTA complexes decompose first
and are immediately precipitated as the oxalates . Hence , the lighter lanthanoids such
as Ce3+ , Pr3+ , Nd3+ are precipitated out first , which are filtered off . The separation
is not complete , so the oxalates are redissolved and the process is repeated many
times .
Change of oxidation states by selective oxidation or reduction : +3 oxidation state
is most characteristic of all the elements of lanthanoid series . Some of the
lanthanoids also show +2 and +4 oxidation states in addition to +3 state . But
the properties of Ln4+ and Ln2+ are so different from those of Ln3+ that their
separation becomes fairly easy .
Cerium can be separated from a mixture of lanthanoids , as it is the only
lanthanoid which has a stable (+4) oxidation state in aqueous solution . Oxidation of
a solution containing a mixture of the lanthanoid ions in +3 oxidation state with
NaOCl , or KMnO4 or chlorine , in alkaline medium , leads to the oxidation of
Ce3+ to Ce4+ .only . Since Ce4+ ion is smaller , less basic and less soluble than any
Ln3+ or Ce3+ ion , Ce4+ can be easily precipitated as Ce(OH)4 by the addition of a
small amount of an alkali to a solution of mixture of lanthanoids .
Eu can be separated from a solution containing various Ln3+ ions by reduction of
Eu3+ to Eu2+ . Zn - amalgam is the main reducing agent and once Eu3+ is reduced to
Eu2+ , the latter can be precipitated as insoluble EuSO4 . It is to be noted that the
sulphates of all the trivalent lanthanoids are soluble in water .
Lanthanoids such as europium , samarium and ytterbium can be removed effectively
from mixtures by their reduction to amalgams . This can be carried out either by
extracting a buffered acidic solution with liquid sodium amalgam or by electrolyzing
an alkaline citrate solution with a lithium amalgam cathode . This method of
separation is convenient , rapid and can be operated continuously on a large scale .
Ion exchange: This is the most effective method now available for the separation
of lanthanoids. The method utilizes synthetic cation exchange resins , which are
generally organic compounds containing sulphonic (- SO3H) or carboxylic acid (COOH) groups . Thus , if an aqueous solution containing a mixture of Ln3+ ions is
allowed to pass down a column packed with a cation exchange resin (represented as
HR(s)) , the Ln3+ ions replaces the equivalent number of H+ ions of the resin and
hence themselves get fixed on the resin . This exchange of Ln 3+ ions in solution with
protons on a solid ion exchanger is an equilibrium process , represented as :
Ln3+(aq) + 3HR (s)
LnR3 (s) + 3H+ (aq)
The tenacity with which a cation is held
decrease in its crystal radii and charge as :
by
an
ion
exchanger
decreases
with
Th4+ > La3+ > Ce3+ > Y 3+ > Lu3+ > Ba2+ >Ca2+ > K+ > NH4+ > H+
Hence , amongst the tripositive lanthanoide ions , La3+ is attached to the resin in the
column with maximum and Lu3+ with minimum firmness . In order to remove the Ln3+
ions , fixed on the resin as LnR3 , the column is eluted with an eluting agent . Generally
, sodium chloride is used as an eluting agent . But for the separation of lanthanoids ,
it is not considered very effective , as it results in a low degree of separation .
However , when eluted with a suitable complexing agent , such as a buffer solution of
ammonium citrate - citric acid and amine polycarboxylates , separation of lanthanoid
ions into individual bands occurs on the column and the species ultimately leave the
column in reverse order of their atomic numbers (Fig 11) . Hence , the fractionating
ability of an ion exchanger is enhanced by a complexing agent . This can be explained
on the basis of the value of separation factor ∝ .
Fig 11 : Graphical representation of the elution order for the lanthanoids
The distribution of a lanthanoid between a solid exchanger (HR or simply r) and an
aqueous phase in terms of equilibrium :
Ln3+(aq) + 3HR(s)
Ln(R)3(s) + 3H+ (aq)
is described as :
λ
=
[Ln3+(r)] / [ Ln3+(aq)]
where λ is the distribution coefficient . For convenience, neglecting the ionic charges, we get
λ
=
[ Ln(r )]
[ Ln(aq)]
Similarly , for a second lanthanide (Ln / )
λ0
=
[ Ln ' ( r )]
[ Ln ' ( aq )]
Thus , separation factor , ∝ =
=
λ0
λ
[ Ln' (r )].[ Ln(aq)]
[ Ln' (aq )].[ Ln(r )]
Under these conditions , the value of ∝ remains almost equal to unity , making the
separation of Ln and Ln/ difficult . However , if a chelating agent (say , AA) is
introduced , the additional equilibria are also established in the solution .
Ln (AA)n(aq)
Ln(aq) + n(AA)(aq)
and ,
Ln/
+
Ln/ (AA)n
n(AA)
The formation constant , K , for the formation of Ln(AA)n can be expressed as ;
K Ln ( AA) n
[ Ln( AA) n (aq )]
=
[ Ln(aq )][( AA)(aq )] n
or , [Ln(AA)n (aq)] = K Ln ( AA) n .[ Ln( aq )].[( AA)(aq )]
n
Hence , in presence of the chelating agent , the total concentration of the lanthanoid ion
in aqueous phase is the sum of the uncomplexed and complexed ion . The distribution
coefficient , λ , (for Ln), thus get modified to λ/ .
[ Ln(r )]
λ/
=
{[ Ln(aq)] + [ln( AA) n (aq )]}
or ,
λ/
=
or,
λ' =
[ Ln(r )]
{[ Ln(aq)] + K Ln ( AA) n [ Ln(aq)][( AA)(aq )] n }
=
[ Ln(r )]
[ Ln(aq)](1 + K Ln ( AA) n [( AA)(aq )]n )
λ
(1 + K Ln ( AA) n [( AA)(aq)] n )
If the formation constant is sufficiently large , the above equation reduces to :
λ'=
λ
K Ln ( AA) n [( AA)(aq)]n
Similarly , the behaviour of other lanthanoid ion , Ln / would be described in terms
of K/ Ln’(AA)n and λ0/ , in presence of a complexing agent .
Hence , the separation factor , ∝/ , in presence of a complexing agent is represented as :
∝/ =
λ ' 0 λ0 K Ln ( AA)
=
λ ' λK ' Ln '( AA)
n
n
Thus , if a complexing agent gives complexes of different stability with different
lanthanoid ions , the value of separation factor will deviate largely from unity , making
the separation of lanthanoid ions easy . This holds good only when the lanthanoid ion
reacts with the complexing agent in 1:1 mole ratio , resulting in the formation of a
single complex species . It is also assumed that the chelated species are of sufficient
stability, so that almost all the lanthanoid ions in the solution exist as complex species .
This is true with chelates such as amine polycarboxylates , but with citrates , metal
chelates of variable compositions are formed , depending upon the pH . A buffer
solution of ammonium citrate – citric acid is thus used so that the pH is sufficiently
high (5.0 – 8.0) and the various equilibria of the type
Ln(AA)m-n + H+
H Ln (AA)n(m-) + 1
can be avoided and only the species [Ln(cit)2]3- is significant . Using this complexing
agent , the separation is rapid , effective , complete and also provides hydrogen ion as a
suitable retaining ion . By this trechnique , large quantities of lanthanoids of purity better
than 99. 99% can be separated . However, it is not important commercially due to its
low capacity.
Other complexing agents such as amine polycarboxylates and ethylenediamine tetraaceteic
acid (EDTA) have also been found to be effective eluants for separating the lanthanoide
ions . Infact , the average separation factor for adjacent lanthanoids is observed to be
2.4 , using EDTA .
Another more easily adapted
technique is “displacement chromatography”. In this
technique, two separate columns of cation exchange resin are employed . The first
column is loaded with a mixture of various tripositive lanthanoid ions and the second
column , known as development column , is loaded with a salt of retaining ion such as
Cu(II) or Zn(II) or Fe(III) . The two columns are coupled together . An aqueous solution
of complexing agents such as triammonium salt of EDTA4- is generally used as eluant .
When this eluant is passed through the first column , the resin bounded lanthanoid ions
(LnR3) are displaced by the equivalent number of ammonium ions .
LnR3 (s) + (NH4)3(EDTA – H) (aq)
(NH4)3 R3 (s) + Ln(EDTA – H) (aq)
With the addition of more and more solution of the complexing agent and with the
removal of products so formed , the reaction progresses in the forward direction . The
solution of Ln(EDTA – H) and (NH4)3 (EDTA – H) reaches the development column ,
where Cu(II) is displaced and Ln3+ is redeposited in a compact band on the top of the
column .
3CuR2(s) +
2Ln (EDTA – H)(aq)
2 LnR3(s)
+
Cu3(EDTA – H)2(aq)
Cu(II) , inspite of its lower charge is able to form a more stable complex with (EDTA –
H)3- , due to its smaller size than Ln(III) . The importance of using Cu(II) should be
understood. Instead of Cu(II) if the resin is loaded with H+ or with citrate which provide
hydrogen ion as a retaining ion, EDTA.H4 will precipitated out and would clog the
resin .
Once the Ln3+ ions have been deposited on the resin , they are displaced again by the
NH4+ in the eluant . The affinity of Ln3+ ions for the resin decreases almost negligibly
with increasing atomic mass and thus the elution of the development column with NH4+
alone is not able to discriminate effectively between different lanthanoids . However,
the value of formation constant for Ln(EDTA-H) complex increases by approximately
25% from Ce3+ to Lu3+ . Thus , in the presence of complexing agent like EDTA4- , the
tendency to leave the resin and go into the solution is significantly greater for
heavier than for the lighter lanthanoides , leading to the concentration of heavier
cations in the solution . The Ln3+ ions pass down the development column in the band ,
being repeatedly deposited and redissolved , concentrating the heavier members in the
solution phase . Thus , when all copper has come off the column , the lanthanoids emerge
in succession , starting from the heaviest . They may then be precipitated from the
eluant as insoluble oxalates and ignited to oxides , if required .
Once the different lanthanoid elements have been separated completely or partially , it is
not easy to obtain pure metals because of their high melting points and ease of
oxidation . The unseparated mixture of La and the lanthanoids is used mainly as misch
metal . Misch metal contains 50% Ce , 40% La , 7% Fe and 3% other lanthanoids , and
is used to strengthen steel . Though , the metals are of little importance of their own , two
important methods namely electrolysis of fused salts and metallothermic treatment of the
anhydrous salts , are available for producing the metals .
Magnetic Properties
The major magnetic properties of any chemical substance result from the fact that each
moving electron is itself a micro – magnet . Since , an electron has both spin and orbital
motion , it may contribute to magnetic behaviour in two ways . The magnetic properties
of a substance thus represent the combined contribution of all the electrons present .
When a substance is placed in a magnetic field , it is observed to align itself either in
opposition to the field (diamagnetic behaviour) or parallel to the field (paramagnetic
behaviour) . Diamagnetism results when there are no unpaired electrons and pairing of all
electrons nullifies their individual contribution . The ions Y3+ , La3+ and Lu3+ are
diamagnetic . Paramagnetism results when unpaired electrons are present to prevent such
compensation . All of the other lanthanoid tripositive ions are paramagnetic .
Let us now consider the contribution of two or more electrons in the outer shell to the
total angular momentum of the atom . There are two different ways in which we might
sum the orbital and spin momentum of several electrons :
(i) First sum the orbital contributions and then the spin contributions separately , finally
add the total orbital and total spin contributions to reach the grand total .
Symbolically: Σml = L , ΣmS = S , L+S = J .
S is the resultant spin quantum number , L is the resultant orbital momentum quantum
number and J is the total angular momentum quantum number .
(ii) Sum the orbital and spin momenta of each electron separately , finally summing the
individual totals to form the grand total : ml +ms = j ; Σj = J
The first method , known as Russell – Saunders coupling , gives reasonably accurate results
for small and medium sized atoms , such as lanthanoids . The second method called j - j
coupling applies better to large atoms. Russell – Saunders coupling shall be discussed in detail
and will correlate it with the calculation of magnetic moment .
The magnetic moment of transition i.e. d – block elements may be calculated from the
expression :µ (S+L) = 4 S ( S + 1) + L( L + 1)
where , µ (S+L) is the magnetic moment in
spin and orbital momentum contributions .
orbital contribution is usually quenched out
electric fields of the ligands . Thus their
only the spin – only formula .
µS = 4 S ( S + 1) = n(n + 2)
Bohr magnetons calculated using both the
For the elements in the 3d - series , the
by interactions of the metal ion with the
magnetic moments can be calculated using
where µS is the spin only magnetic moment in Bohr magnetons and n is the number of
unpaired electrons . This simple spin only formula works with La3+(f0) , Gd3+(f7) and
Lu3+ (f14) . La3+ and Lu3+ have no unpaired electrons i.e. , n = 0 and hence
µS= 0(0 + 2) = 0 . Gd3+ has seven unpaired electrons , n = 7 and µS = 7(7 + 2) = 63
= 7.9 BM . The other lanthanoid ions , however , do not obey this simple relationship .
The 4f electrons are deeply placed and shielded from external fields by the overlying 5s2
and 5p6 shells. Thus , the magnetic effect of the motion of the electron in its orbital
is not quenched out . The magnetic moments , therefore , are calculated taking into
account both the magnetic moment from the unpaired electron spins and that from the
orbital motion . This is true not only for lanthanoids , but also for the elements
belonging to the 4d and 5d series .
In principle , the electronic configurations of the lanthanoids are described by using the
RUSSELL – SAUNDERS coupling scheme as follows –
(i) Term with highest S and highest L lies lowest .
Hence , for Nd3+ (4f 3 configuration) , the lowest energy arrangement is :
ml
3
2
Thus , L = Σml = 3 + 2 + 1 = 6
1
0
-1
-2
-3
S = ΣmS = 3 x 1/2 = 3/2
The value of L is then expressed for each ion in the form of a ground term with
the symbolism extended so that S , P , D , F , G , H , I-------, correspond to L = 0 , 1 ,
2 , 3 , 4 , 5 , 6------ , in that order . The term symbol is written as 2S + 1L . For f 3
configuration , it is 4I . At this point , however , a crucial difference emerges between
the lanthanoid and 3d element . Since the d electrons of the transition metal ions are
exposed directly to the influence of the neighbouring groups , the effect of the crystal
field on the free - ion ground terms has to be considered before that of spin - orbit
coupling . But the 4f electrons of the lanthanoid ions are deeply buried in the inner
electron core and are very effectively shielded from their chemical environment . The
spin contribution S and orbital contribution L couple together to give a new quantum
number J called the total angular momentum quantum number . This coupling (mostly of
the order of 2000 cm-1) , is , therefore , much larger than the crystal field (∼ 100 cm-1) and
its effect must be considered first . J can take the following values :
J = L+S , L+S-1-------- L-S
or , the ground state of the ion is that with
J = L – S (when the shell is less than half full)
and J = L + S (when the shell is more than half full)
The complete term symbol is represented as
symbol for Nd3+ is 4I9/2 .
2S+1
LJ and thus , the ground state term
Similarly , the ground state term symbol for Tb3+ with f8 configuration can be calculated
as follows :ml
3
2
1
0
-1
-2
-3
L = 3 , S = 6 X 1/2 = 3 , J = 6 , 5 , 4 , 3 , 2 , 1 , 0
Since it is more than half filled , for ground state the value of J = L + S = 6 . Hence ,
for f8 configuration , the ground state term symbol is 7F6 . The magnetic moment µ is
calculated by using Lande’s formula as :
µ = g J ( J + 1)
where , g =
3 S ( S + 1) − L( L + 1)
+
2
2 J ( J + 1)
Due to a large spin – orbit coupling the lanthanoid ions , with a few exceptions , have
ground state with a single well - defined value of the total angular momentum quantum
number J , with the next lowest J state at energies many times above and hence virtually
unpopulated . Fig 12 shows the calculated magnetic moments for the lanthanoid ions using
both the simple spin only formula and the coupled spin – orbital momentum formula .
For most of the lanthanoide ions , there is excellent agreement between the calculated
and the experimental values. However, the agreement for Eu3+ and Sm3+ is poor (table 10 )
because of the fact that for Sm3+ and Eu3+ , the spin – orbit coupling is only about 300 400 cm-1 . This means that the difference in energy between the ground state and the
next state is small . Hence , the energy of thermal motion is sufficient to promote some
electrons to the first excited state (and in the case of Eu3+ even to the second and the
third excited state) . Due to this , the magnetic properties are not solely determined by
the ground state configuration . Since these excited states have higher J values than the
ground state , the actual magnetic moments are higher than those calculated by
considering the ground states only . However , magnetic moments of Eu3+ measured at
low temperatures give a value close to zero , as expected . The electronic configuration
suggested for the non – tripositive species are in agreement with magnetic data . Thus ,
Ce(+4) and Yb(+2) are diamagnetic , corresponding to 4f 0 and 4f 14 configurations ,
Sm(+2) and Eu(+3) are magnetically similar (both 4f6) and Eu(+2) shows similar to
Gd(+3) (both 4f7) .
Fig. 12 : Paramagnetic moments of Ln3+ lanthanoid ions at 300 K .
Table 10 : Magnetic moments of Ln3+ ions .
Number of 4f
electrons
La3+
Ce3+
Pr3+
Nd3+
Pm3+
Sm3+
Eu3+
Gd3+
Tb3+
Dy3+
Ho3+
Er3+
Tm3+
Yb3+
Lu3+
[Xe]4f0
[Xe]4f1
[Xe]4f2
[Xe]4f3
[Xe]4f4
[Xe]4f5
[Xe]4f6
[Xe]4f7
[Xe]4f8
[Xe]4f9
[Xe]4f10
[Xe]4f11
[Xe]4f12
[Xe]4f13
[Xe]4f14
Magnetic Moment
Calculated (BM)
Observed (BM)
0
2.54
3.58
3.62
2.68
0.84
0
7.94
9.72
10.63
10.60
9.57
7.63
4.50
0
0
2.3 - 2.5
3.4 - 3.6
3.5 - 3.6
2.7
1.5 - 1.6
3.4 - 3.6
7.8 - 8.0
9.4 - 9.6
10.4 - 10.5
10.3 - 10.5
9.4 - 9.6
7.1 - 7.4
4.4 - 4.9
0
Several of the paramagnetic lanthanoid ions , especially Pr3+ , Eu3+ and Yb3+ are useful
as NMR shift reagents . When an organic molecule with a complex NMR spectrum is
coordinated to one of these ions , the large magnetic moment of the ion causes
displacements and a spreading out of the spectrum . This often helps in assigning and
interpreting the peaks .
Colour and Spectra
Electronic absorption spectra are produced when electromagnetic radiation promotes the
ions from their ground state to the excited state . Many trivalent lanthanoid ions are
strikingly coloured , both in the solid state and in aqueous solution . Interestingly , the
colours from the ions from La3+ through Gd3+ repeat themselves , atleast qualitatively
from Lu3+ back through Gd3+ (table 11 ) . It is tempting to conclude , that a given colour
is related to a given number of unpaired electrons and the elements with nf electrons
often have a colour similar to those having (14 - n) f electrons . However , the
divergence in colour between the tri – positive species and isoelectronic non - tripositive
ions (table 12 ) suggests that the situation , in reality , is somewhat more complex .
Table 11 : Colour of Ln3+ ions
La3+
Ce3+
Pr3+
Nd3+
Pm3+
Sm3+
Eu3+
Gd3+
Tb3+
Dy3+
Ho3+
Er3+
Tm3+
Yb3+
Lu3+
Electronic
Ground state
configuration
[Xe]4f0
[Xe]4f1
[Xe]4f2
[Xe]4f3
[Xe]4f4
[Xe]4f5
[Xe]4f6
[Xe]4f7
[Xe]4f8
[Xe]4f9
[Xe]4f10
[Xe]4f11
[Xe]4f12
[Xe]4f13
[Xe]4f14
1
Number of
unpaired electrons
S0
F5/2
3
H4
4
I9/2
5
I4
6
H5/2
7
F0
8
S7/6
7
F6
5
H15/2
5
I8
4
I15/2
3
H6
2
F7/2
1
S0
2
0
1
2
3
4
5
6
7
6
5
4
3
2
1
0
Colour
Colourless
Colourless
Green
Lilac
Pink
Yellow
Pale pink
Colourless
Pale pink
Yellow
Pale yellow
Pink
Pale green
Colourless
Colourless
Table 12 : Colour of Ln2+ , Ln4+ and their isoelectronic Ln3+ counterparts
Non-tripositive Colour
ions
Ce4+
Sm2+
Eu2+
Yb2+
Orange-red
Blood-red
Straw yellow
Yellow
Electronic
configuration
[Xe]4f0
[Xe]4f6
[Xe]4f7
[Xe]4f14
Colour
Tripositive ion
Colourless
Pale pink
Colourless
Colourless
La3+
Eu3+
Gd3+
Lu3+
The ions Ce3+ , Gd3+ , Yb3+ , all of which contain unpaired electrons are colourless along
with the lanthanoid ions having no unpaired 4f electrons (La3+ with 4f0 and Lu3+ with
4f14 configuration) . Colour arises because light of a particular wavelength is absorbed
in the visible region . Tripositive ions except Y3+ , La3+ and Lu3+ absorb in the
wavelength range 2000 – 10000 A0 . The colourless species absorb either in ultraviolet
(Ce3+ , Gd3+) or the infrared region (Yb3+) . The dipositive ions absorb strongly in the
ultraviolet region . The only tetrapositive ion stable in aqueous solution , the Ce4+ ion ,
absorbs in the blue and ultra - violet regions .
In the lanthanoids , spin – orbit coupling is more important than crystal field splitting .
This is in contrast to the transition metals, where the crystal field splitting is of major
importance and the crystal fields even lift some of the orbital degeneracy of the terms
of dn ions . Nevertheless , crystal fields can not be completely ignored for lanthanoide
ions . The intensity of a number of bands show a distinct dependence on the actual
ligands which are coordinated . Also , the crystal fields lift up some of the orbital
degeneracy of the states of fn ions ( by the order of 100 cm-1 ). Thus , when the
electronic transitions , called f - f transitions , occur from one J state of an fn
configuration to another J state of this configuration , the absorption bands are extremely
sharp . The colour remains unaffected by alternation of the ligands present or addition
of colourless complexing groups . They are apparently characteristic of the cation
themselves . Strictly , the f - f transitions are Laporte forbidden (since the change in the
subsidiary quantum number is zero i.e. ∆ml = 0) . Moreover , the vibration in the metal ligand bond is very small leading to only a slight relaxation in the Laporte rule .
Hence , the transitions are not very intense , giving only pale colours to the complexes .
This is in marked contrast to the transition elements where d - d spectra gives
absorption bands whose position depends on the number and nature of the ligand and
the width of the peak is greatly broadened because of the vibration of the ligands .
Though d - d transitions are also Laporte forbidden , but these are partially relaxed by a
mechanism which depends on the effect of the crystal field in distorting the symmetry of
the metal ion . Hence , the transition metal complexes have more intense colour than the
lanthanoids .
Ce3+ (4f1) and Tb3+(4f8) are exceptional in providing bands of appreciably higher
intensity in the ultra - violet region . The absorption is very intense for two reasons .
Firstly , the transitions involved are of the type 4fn
4f n-1 5d1 i.e. 4f to 5d and hence
∆ml =1 , making them orbitally allowed , as against forbidden f – f transitions .
Furthermore , promotion of electrons in these ions is easier than for other ions . The
electronic configuration for Ce3+ is 4f1 and Tb3+ is 4f8 . Loss of one electron gives the
extra stability of an empty or half – full shell . The absorption bands of the Ce3+ and
Tb3+ ions are broad and are altered by complexing groups .
The spectra of Ln2+ ions , stabilized in CaF2 type crystal can be studied . It is expected
that their spectra would resemble those of the +3 ions of the next element in the
series . However , due to lower ionic charge of the Ln2+ ion , their 4f orbitals have not
been stabilized relative to the 5d to the same extent as those of the Ln3+ ions .
Therefore , the spectra of Ln2+ consist of broad , orbitally allowed , 4f
5d bands
with much weaker and sharper f - f bands . These dipositive ions absorb strongly in the
ultra – violet region .
Charge transfer spectra are also possible along with the f - f and f - d transitions . The
charge transfer spectra arise due to the transfer of an electron from the ligand to the
metal . Such spectra are more intense than those obtained due to f – f transitions .
Charge transfer is more probable if the metal is in a high oxidation state or the ligand
has reducing properties . The orange - red colour of Ce4+ solutions arises from charge
transfer rather than f - f spectra . Similarly , the blood - red colour of Sm3+ is also due to
charge transfer .
The
unique absorptions associated with the f electrons make some of the lanthanoids
particularly useful in light filters . Glass containing praseodymium and neodymium ions
(known as didymium glass) absorb yellow sodium light so strongly that it is used
mainly in glass blower’s goggles . The sharp and characteristic absorption spectra of Ln3+
are used extensively for both the qualitative and quantitative analysis of the lanthanoids
in mixtures . Calibration of wavelength in optical devices is done by taking the
advantage of the sharpness of these absorption bands .
Fluorescence or luminescence of certain lanthanoid ions , notably Tb , Ho and Eu is
another important feature . Luminescence is the emission of light by a material as a
consequence of its absorbing energy . It is known as PHOTO LUMINESCENCE , if
photons are used for excitation . A photo luminescent material generally requires a host
(crystal structure) doped with an activator along with a sensitizer . Luminescence with a
short time lapse (∼ 10-8 s ) between excitation and emission is known as FLUORESCENCE .
However , if luminescence continues for long even after the removal of excitation source
and the decay time is very long , it is known as PHOSPHORESCENCE . These
phenomenon are commercially very important . For example , red phosphor in TV tubes
is obtained from Eu3+ doped into Y2O3 . Other luminescence colours are : green ( from
Ce3+ doped in CaS or Eu3+ doped in SrGa2Su ) , blue ( from Eu2+ doped in (SrMg)2)
.P2O7 ) . Mixing red , green and blue is effective in giving white luminescence , used in
fluorescent lamp coatings to convert UV / blue discharge to white light . The luminescence
of Eu3+ is used to probe its environment , giving information on ligand charges , binding
constants , ligand exchange rates , site symmetry etc. Further , Ln3+ can be used as a
probe for Ca2+ sites in bioinorganic chemistry . Since , the ionic radii and coordination
number of Ln3+ are close to Ca2+, Ln3+ may replace Ca2+ in its binding sites in proteins .
Chemistry of “0” Oxidation State – The Metal
Yttrium and the elemental lanthanoids are all metals , as might be expected from their
electronic configurations , large atomic radii and their position in the periodic table .
However ,
ease of oxidation of the metals suggests that reduction of ions to the
metallic state is difficult . In aqueous solution , neither electrolytic nor chemical reduction
is effective . Electrolysis of molten halides and metallothermic treatment of anhydrous
salts constitute successful reduction systems.
(i) Electrolysis of molten halides : A mixture of LnCl3 with either NaCl or CaCl2 is
fused and electrolysed in a graphite lined steel cell which serves as the cathode and the
graphite rod as anode . This method is mainly used for misch metal , for Ce , Sm , Eu
and Yb .
(ii) Metallothermic reduction : La and the lighter metals are obtained by reducing
anhydrous chlorides with Ca at 1000 – 1100оC in an argon filled vessel .
1000 - 1100оC
2 LnCl3
+
3 Ca
2 Ln
+
3 CaCl2
( Ln = La , Ce , Pr , Nd , Gd )
The heavier lanthanoids have higher melting points and hence require a temperature of
1400оC . However , at this temperature CaCl2 starts boiling . Thus , LnF3 are used
in place of LnCl3 .
2 LnF3
+
3 Ca
2 Ln +
3 CaF2
( Ln = Tb , Dy , Ho , Er ,Tm , Y )
A mixture of LnF3 and Ca is heated in a tantalum crucible to a temperature 50о - 100 оC
above the melting point of Ln under an inert atmosphere . The charge is then cooled
and the slag and metal is broken apart . The impurities of Ca are removed by melting
under vaccum .
Promethium is obtained by reducing PmF3 with Li
PmF3
+
3 Li
Pm
+
3 LiF
Trichlorides of Eu , Sm and Yb are reduced only to the dihalides by calcium , but the
metals can be prepared by reduction of the oxides Ln2O3 with La at high temperatures .
Ln2O3
+
2 La
(Ln = Sm , Eu , Yb )
2 Ln
+
La2O3
The metals are best purified by distillation in tantalum apparatus with a vaccum of
atleast 10-5 mm of Hg .
The metals are silvery – white and are very reactive . They are relatively soft with
hardness increasing with atomic number . They have high density , and melting and
boiling points which increase with increase in the atomic mass . Eu and Yb have lower
values of melting and boiling points , which probably reflect the differences in crystal
structure . Their compounds are generally ionic . They are strong reducing agents and
react readily with most non - metals to form binaries . They are malleable and ductile .
These elements also exhibit the nuclear property of capturing the neutrons . Cerium is a
weak neutron absorber , but Sm , Eu and Gd are more effective than boron or cadium the substances normally used in neutron control devices . Europium is of particular
interest because its isotopes undergo a series of n , γ reactions , each of which gives a
europium isotope of large absorption cross sections . In electrical conductivity , they cover
the same range that cesium and mercury do . Most of the metals are paramagnetic and
gadolinium is unique in being ferromagnetic , but upto 16оC only .
The occurrence of reactions of the type :
Ln (s)
+
xH2O
Ln (H2O)x3+
+
3e-
(acidic medium )
+
3 OH-
Ln (OH)3 (s)
+
3e-
(basic medium )
and
Ln (s)
indicate their ease of oxidation under aqueous conditions . The metals are strong reducing
agents under these conditions due to the large amounts of energy released in the
hydrolysis of their gaseous tripositive ions as compared to the smaller quantities of
energy needed to form these ions . Even in absence of water the metals are readily
oxidized , though slowly at room temperature and rapidly at high temperatures .
The lanthanoids are much more reactive than Al (E0 = - 1.66 V) and are slightly more
reactive than Mg (E0 = - 2.37 V) . Thus , they all react directly with water , slowly in
the cold , rapidly on heating , to liberate hydrogen and forming insoluble hydroxides or
hydrous oxides . Eu is attacked much more rapidly than other metals , forming first the
soluble , yellowish Eu(OH)2.H2O and then hydrous Eu2O3 . The hydroxides are ionic
and basic .
Dry oxygen attacks pure metals very slowly at room temperature , but at higher
temperatures the metals ignite and burn . They tarnish readily in moist air and burn to
give the sesquioxides , Ln2O3 except cerium which gives CeO2 . Yb and Lu form
protective oxide films , on exposure to moist air , which prevents the bulk of the metal
forming the oxide unless it is heated to 10000 C . Praseodymium and terbium yield non –
stoichiometric products , Pr6O11 and Tb4O7 , respectively .
Chalcogenides of the type LnZ and Ln2Z3 ( Z = S , Se , Te ) are formed by the direct
combination with the respective lanthanoid ( except Pm ) . Their characterization is made
more difficult by the prevalence of non – stoichiometry . In general , they are stable in
dry air but are hydrolysed if moisture is present .
The metals react exothermically with hydrogen , though heating to 300 - 4000C is often
required to initiate the reaction . The resulting phases MH2 and MH3 , which are usually
in a defect state , have remarkable thermal stability , in some cases even upto 9000C .
The metals also react readily with C , N , Si , P , halogens and other non – metals at
elevated temperatures . Reactions of the metals with typical reagents are summarized in
Table 13 .
Like alkali and alkaline earth metals , metallic Eu and Yb dissolve in liquid ammonia
at -780 C to give a dark blue solution . The solutions are strongly reducing , decompose
slowly and are believed to contain the ammoniated electron , e(NH3)-y . The solution
changes to golden on concentration .
Table 13 : Typical chemical reactions of the elemental lanthanoids
Reagent(s)
Product(s)
H2
X2(X= F, Cl, Br, I)
LnH2, LnH3
LnX3
S
N2
C
Ln2S3
LnN
LnC2, Ln2C3 (also LnC, Ln2C,
Ln3C, Ln4C)
LnB4, LnB6
Ln2O3
B
O2
Conditions
Dil. Acids(HCl, H2SO4,
HClO4)
H2O
Ln3+ + H2
H2O + O2
Ln2O3 or Ln(OH)3
Metal oxides
metal
Ln2O3 or Ln(OH)3 + H2
Rapid above 3000C
Slow at room temperature,
burn above 2000C
At boiling point of S
Above 10000C
At high temperature
At high temperature
Slow at room temperature,
burn above 150-1800C
Rapid at room temperature
Slow at room temperature,
more rapid at high temperature
Rapid with Eu, slow with
others
At high temperatures (except
CaO, MgO, Ln2O3 in general)
Chemistry of Lanthanoid Ions in +2 Oxidation State
All lanthanoids can give Ln2+ ions under unusual conditions . For example , reduction of
tri –chlorides of Nd , Dy or Tm by sodium naphthalenide or reduction of tri - bromides
and tri - iodides of Sm , Dy , Tm and Yb by alkali metals (M/ ) give M/ LnX3 . Though
the reduction potentials for these ions (in THF) are known [ Tm2+ (-2.3 V) , Dy2+(-2.5 V)
, Nd2+(-2.6 V )] , only Sm2+ , Eu2+ and Yb2+ have significant normal aqueous chemistry
in ( + 2 ) state . The most stable divalent lanthanoid is Eu2+, which is highly stable in
water . Both Sm2+ and Yb2+ are rapidly oxidized by hydronium ions :
2 Ln2+
+
2 H3O+
2 Ln3+
+ 2 H2O
+ H2 (g)
but the Eu2+ ion is oxidized only very slowly . All the three ions are oxidized rapidly
in the presence of oxygen .
4 Ln2+
+
4 H3O+
+
O2
4 Ln3+
+
6 H2O
The general instability of the dipositive species with respect to oxidation , in aqueous
medium has already been discussed . However , in less strongly solvating media , such as
alcohol , some stabilization with respect to oxidation may be expected and is best
achieved by including these ions in solid compounds . More extensive stabilization is
noted when the ions are trapped in an SrO , Ln2+ in silicate glasses , LnF2 in LnF3 .
The divalent lanthanoids are generally obtained by reduction of anhydrous fluorides or
chlorides or fused tri – halides or oxides with the corresponding metal . Eu2+SO4 can be
prepared by electrolyzing Eu23+(SO4)3 . Amalgamated Zn can reduce Eu(+3) to Eu(+2) ,
but Sm(+3) and Yb(+3) can not be reduced by this method . Electrolytic reduction of
aqueous solutions of Eu(+3) and Yb(+3) yields the respective dipositive ion at the
mercury cathode . But Sm(+2) ion is too strongly reducing to be so prepared . Sm , Eu
and Yb amalgams , on treatment with acids , apparently give the dipositive ions as
intermediate oxidation products . The best way of obtaining these dipositive ions is by
thermal decomposition of the trihalides at high temperature :
high temperature
2LnX3(s)
2LnX2(s)
+
X2(g)
The ease of reduction increases in the series Sm3+ - Yb3+ - Eu3+ and Cl- - Br- - I- .
SmI2 and YbI2 can be prepared by reacting the metal with 1, 2 – diiodomethane in
anhydrous THF .
Ln
+
ICH2CH2I
LnI2
+
CH2=CH2
There are striking similarities in the crystal structure and solubility between these
divalent ions and the alkaline earth metals , especially Sr2+ and Ba2+ . All dipositive
fluorides are isomorphous , having the fluorite type structure . Anhydrous SmCl2 is
isostructural with SrCl2 and BaCl2 . EuBr2 and SmBr2 are isostructural with SrBr2 . It is
for this reason that the Sr2+ and Ba2+ ions are commonly used to carry the dipositive
lanthanoids in separations . In low oxidation state , iodides are more numerous than
fluorides . In solid state , even the diiodides of Dy , Tm and Yb are known to form layer
structures like CaI2 . Most of the dihalides are air and moisture sensitive .
Eu (+2) resembles Ca (+2) in several ways. Both form insoluble sulphates and
carbonates and are soluble in liquid NH3 . EuH2 is ionic and similar to CaH2 . The
major difference between the two is that Ca compounds are diamagnetic whereas EuX2
have a magnetic moment of 7.9BM , corresponding to seven unpaired electrons . The
halides of Ln(II) are finding increasingly more uses . SmI2 is considered as a highly
versatile reagent in organic chemistry . It can be used in the reduction of organic
functional groups ( i.e. R - X can be converted to R - H) . It can also be used in the
formation of carbon - carbon bond forming reactions , such as in Barbier or Reformatsky
or Pinacol coupling type of reactions .
Apart from the insulating dihalides , conducting diiodides of Ce , Pr and Gd are also
known . They are known for their metallic lusture and high conductivities . They are best
formulated as Ln3+ , 2I- , e - , the electron being delocalized in conduction band . The
observed isomorphism between the compounds TmI2 and YbI2 suggests that the Tm2+
exists . Apart from this , other reduced phases such as Smx+5Br13 (x = 2.20) , Smx+11Br24
(x = 2.18 ) and Smx+6Br13 (x = 2.16) have also been identified . They have fluorite - like
structure in which the anions are rearranged to accommodate the additional anions ,
giving an irregular 7 - and 8 - coordinated lanthanoid ion . Gd 5 Cl11 and Ho 5 Cl11 have
also been characterized . Further reduction, below the + 2 oxidation state in LnCl3 / Ln
melt gives rise to Ln2Cl3 and finally “graphite - like” LnCl phase .
Dioxides of the type NdO , SmO ( both golden yellow ) , EuO (dark red) and YbO
(greyish white) are known . They are obtained by reducing Ln2O3 with the metal at high
temperature , except for Eu , which requires high pressure . EuO and YbO
are
insulators or semiconductors and have NaCl like structure . EuO is found to be
ferromagnetic at low temperatures . However , NdO and SmO consist of Ln3+ , O2- and
an e- in the conduction band .
Monochalcogenides , LnZ (Z = S , Se , Te) , have been prepared for all the lanthanoids
except Pm , mostly by direct combination . They , like oxides , have rock salt - like
structure , and are mostly black . Except SmZ , EuZ , YbZ , TmSe and TmTe , all of
them exhibit conductivity , illustrating the fact that they consist of Ln3+ and Z2- ions , with
an electron in the conduction band . EuZ and YbZ are semi –conductors or insulators
with Ln2+ ion . SmZ seems to be intermediate between Sm2+ and Sm3+ involving the
equilibrium
Sm2+ +
Z2-
Sm3+ +
Z2-
+
e-
The hydrated water - soluble Sm(+2) and Yb(+2) salts are oxidized by their water , but
the salts of Eu(+2) are comparatively stable . Water insoluble compounds (e.g. the
sulphates , carbonates , fluorides) resist oxidation even in the presence of water .
Chemistry of Lanthanoid Ions in +3 Oxidation State
It is apparent from previous discussion that the properties of this state very largely
determine the chemistry of lanthanoids . The tri - positive species are found in crystalline
compounds containing essentially all known anionic species . Compounds of Ln3+ cation
with the anions such as OH- , CO32- , SO42- , C2O42- and NO3- decompose on heating ,
giving basic salts and finally the respective oxides . Hydrated salts that contain thermally
stable anions such as F- , Cl- , Br- and PO43- also give similar products on heating
because of hydrolysis . However, their anhydrous compounds melt without decomposition .
Their high melting points and electrical conductivities in the fused state reflect the high
degree of ionic bonding .
The hydroxides , Ln(OH)3 are definite compounds and may be obtained in crystalline
form by ageing Ln2O3 in strong alkali at high temperature and pressure . They can also
be precipitated out by adding alkali or ammonium hydroxide to a solouble lanthanum
salt such as
chloride , nitrate etc . They have hexagonal structures with nine coordinated tricapped trigonal prismatic geometry . These hydroxides are nearly
insoluble in water but are sufficiently basic to dissolve readily in acids . They are less
basic than Ca(OH)2 but more basic than Al(OH)3 . However , their basicity decreases as
the ionic radius decreases from Ce to Lu . Thus Ce(OH)3 is most basic and Lu(OH)3 is
least basic . The decrease in basicity is evident by the fact that the hydroxides of latter
elements dissolve in hot concentrated NaOH forming complexes .
Yb(OH)3 + 3NaOH
3Na+ + [Yb(OH)6]3-
Lu(OH)3 + 3NaOH
3Na+ +
[Lu(OH)6]3-
Paralleling this change , the [Ln(H2O)x]3+ ions are subjected to an increasing tendency to
hydrolyse and hydrolysis can be prevented by the use of increasing acidic solution .
All lanthanoids , except Eu and Yb form hydrides of the type LnH2 simply by heating
the lanthanoid directly with H2 at a high temperature of 3000 - 3500C . These hydrides are
black , metallic and conduct electricity . They have the fluorite type structure. These
hydrides are better formulated as Ln3+ , 2H- and an electron which is delocalized in a
metallic conduction band . More hydrogen can be accommodated in the interstices of
the lattice and a limiting stoichiometry of salt like hydride , LnH3 can be achieved if
high pressures are employed . The composition of LnH3 is Ln3+ and 3H- , with no
delocalized electrons and hence no metallic conduction . In addition , Yb forms non –
stoichiometric compound approximately to YbH2 . 5. The hydrides are remarkably stable to
heat often upto 9000C . They are decomposed by water and react with oxygen .
CeH2 +
2H2O
CeO2 + 2H2
Due to the low boiling point , low density and explosion risk involved , it is difficult to
store and transport hydrogen fuel . The lanthanoids can form intermetallic alloys of the
type LnNi5 , which can reversibly absorb large quantities of hydrogen . The intermetallic
hydrides of lanthanoids which are capable of storing and producing large amount of ultra
pure hydrogen , can be used as a source of fuel for motor vehicles , in fuel cells , as
well as hydrogenating agent in organic chemistry . An intermetallic alloy of the type
LaNi5 crystallises in CaCu5 type structure , having 9 interstitial sites for H .
∆H = - 31 kJmol-1
LaNi5
+ 3 H2
∆H =
31 kJmol-1
LaNi5H6
The absorption and desorption of hydrogen occurs topotactically , without making any
drastic change in structure but expanding the lattice .
The oxides , Ln2O3 are obtained by heating metal in oxygen or by the thermal
decomposition of the Ln(OH)3 or oxy salts such as Ln2(CO3)3 and Ln(NO3)3 . These are
well characterized . However , Ce , Pr and Tb form stable oxides as CeO2 , Pr6O11 and
Tb4O7 , from which sesquioxides can be obtained by controlled reduction with H2 . These
oxides are generally pale in colour , are strongly basic and have properties resembling
those of the oxides of alkaline earth metals . They readily absorb water and CO2 from
air to form hydroxides and carbonates respectively . Ln2O3 adopts three structure types
conventionally classified as:
A - type : This type consists of LnO7 units , approximate to capped octahedral geometry
(Fig. 13). This structure is favoured by the lightest lanthanoids .
B - type : This type also consists of LnO7 units but in this case two are capped
trigonal prisms and one is a distorted octahedron ( Fig. 14). This type is favoured by the
middle lanthanoids .
C- type: This type is related to fluorite structure and is favoured by middle and heavy
lanthanoids. They have a complex structure with LnO6 units which are not octahedral (
Fig 15 ).
A – type structure
B - type structure
C- type Structure
Fig. 13-15 : Geometry of Ln2O3
Trivalent chalcogenides of the type Ln2Z3 ( Z = S, Se , Te ) with stoichiometry similar
to oxides can be obtained by a variety of methods including direct combination . Direct
action of H2S on the chloride or oxide results in the formation of respective sulphides . On
heating with excess of chalcogen in a sealed tube at ∼6000C , products with composition
up to or nearing LnZ2 are obtained . They seem to be polychalcogenides.
Except Promethium , all lanthanoids form trihalides of the type LnX3 . The trifluorides
are of particular importance because of their insolubility. They can be precipitated
as LnF3. ½ H2O by the action of HF on aqueous Ln(NO3)3 . However , the fluorides
of heavier lanthanides are sparingly soluble in HF due to the formation of soluble
complex , [LnF(H2O)n]2+ .
Aqueous solutions of other trihalides are obtained by dissolving the oxides or carbonates
in aqueous HX . Hydrated salts with 6 or 8 water molecules can then be crystallized
though with difficulty due to their high solubilities . The chlorides are non – volatile ,
deliquescent solids , soluble in H2O and alcohol and crystallize with six or seven
molecules of water of crystallization . Bromides and iodides are rather similar to the
chlorides . Iodides of the first few lanthanoids are orthorhombic while those of the
remaining lanthanoids are hexagonal .
Heating of hydrated halides leads to the formation of
dehydration of the salt
oxohalides
instead of the
heat
LnCl3 . 6 H2O
LnOCl
+
5 H2O
+
2 HCl
The bromides and iodides are even more susceptible to hydrolysis and dehydration .
However, heating CeX3. (H2O)n results in the formation of CeO2 .
The anhydrous trihalides can be obtained by heating the metal and halogen , or by heating
theb oxide with the appropriate ammonium halide .
3000C
Ln2O3
+ 6 NH4Cl
2 LnCl3
+ 6 NH3 +
3 H2O
They can also be prepared as methanolates, LnCl3. 4 CH3OH by treating the hydrates with
2,2 - dimetheoxypropane or as etherates , LnCl3(ether)n by reaction of the oxides or
carbonates with HCl produced in - situ from SOCl2 and H2O in the presence of glyme
(MeOCH2CH2OMe) under mild conditions . These anhydrous trihalides are ionic , high
melting crystalline deliquescent substances . The coordination number of Ln3+ changes
with the radii of the ions . It varies from 9 for LnF3 of the large lanthanoids to 6 for
the LnI3 of the smaller lanthanoids . The trihalides react with glass giving oxohalides
(LnOX) at high temperatures .
high temperature
2 LnX3
+
SiO2
2LnOX
+ SiX4
Lanthanoid salts of most oxy acids such as sulphates , nitrates , perchlorates and
bromates are also known . These salts are readily obtained by dissolving the oxide in
acid and crystallizing . Their aqueous solutions furnish hydrated cations , [Ln(H2O)n]3+
which tend to undergo slight hydrolysis in aqueous solution
[Ln(H2O)n ]3+
+
H2O
[Ln(H2O)n
2+
- 1(OH)]
+
H3O+
The tendency of the [Ln(H2O)n]3+ ion to hydrolyze increase with decreasing ionic radii ,
on moving from La3+ to Lu3+ . Lanthanoid salts with common anions frequently contain
species like Ln(H2O)93+ , which have a tricapped trigonal prismatic (ttp) structure . These
hydrated lanthanoid ions usually have high primary hydration numbers varying from 8
to 9 . The increased polarization of the primary hydration sphere by a smaller cation
enhances further the hydrogen bonding to water molecules leading to the formation of a
secondary hydration sphere . Polymeric species such as [ Ln2(OH)3]3+ and [Ln3(OH)3]6+
may also be present.
Lanthanoid compounds form a large number of double salts like double nitrates such
as 2 Ln(NO3)3. 3 MII(NO3)2. 24 H2O; (MII = Mg, Zn, Ni, Mn ) and Ln (NO3)3 .
NH4NO3 .4 H2O and double sulphates such as Ln2(SO4)3 . 3 M2ISO4 .12 H2O ; (MI = alkali
metal) . On the basis of solubility in Na2SO4 , the double sulphates are categorized into
two groups : cerium group (Ln = La to Eu) which are only sparingly soluble and
yttrium group (Ln = Gd to Lu) which are highly soluble . Since the double salts
crystallize well , these are used to separate the rare - earths from one another .
All lanthanoids are quantitatively precipitated as oxalates from Ln3+ solution containing nitric
acid and oxalate , (ox2-) ion . The insolubility of oxalates in acidic medium ( pH 4 or
less) is very important . Precipitation of oxalates under these conditions may be used to
separate the tripositive lanthanoids from all other cationic species ( excepting the tri - and
tetrapositive actinoids ). The precipitate, on drying and ignition, gives Ln2O3. The nature of
the oxalate precipitated depends on the reaction conditions. In nitric acid and in presence of
ammonium oxalate double salts of the type NH4Ln (ox)2. y H2O( y = 1 or 3) are obtained .
However , in neutral solution , ammonium oxalate gives the normal oxalate with lighter
lanthanoids and mixtures with heavier lanthanoids. The phosphates are insoluble but
become soluble in presence of dilute acid solution.
Carbonates of the type Ln2(CO3)3 are well known.They can be prepared by passing CO2
into an aqueous solution of Ln(OH)3 or by adding Na2CO3 solution to Ln3+ salt solution .
They are best prepared by hydrolysis of chloroacetates
2 Ln(Cl3CCO2)3 +
(n + 3) H2O
Ln2 (CO3)3.n H2O + 3 CO2 + 6 CHCl3
Most of the carbonates are basic , insoluble in water but dissolve in acids with liberation
of CO2 .
An interesting aspect of the chemistry of lanthanoids in + 3 oxidation state is the
solubility of their salts . Water soluble Ln(+3) compounds include the chlorides ,
bromides , iodides, nitrates, acetates , perchlorates , bromates and a number of double
nitrates . Water insoluble compounds include the fluorides , hydroxides , oxides , carbonates ,
chromates , phosphates and oxalates and the salts of most di - and trinegative anions .
The normal sulphates vary from very soluble to difficultly soluble . Ideally , the trend in
solubility for a given salt should be directly related to crystal radii . However , the
experimental observations indicate that the solubility may decrease with decreasing radius
(e.g , OH- ) , increase with decreasing radius (e.g , double magnesium nitrates ) , or change
irregularly (e.g. , SO42- , BrO3- ) . No explanation has been offered for these observations.
Table 14 records the general trends in the solubilities of Ln (+3) salts in water .
The lanthanoids also form well defined compounds with non - metals and metalloids .
They react directly with B at elevated temperatures to give LnB4 and LnB6 . LnB6 are
the most important type of borides , better formulated as Ln3+(B 62- ) (e-) , excepting EuB6
and YbB6 which are represented as Ln2+(B62-) and do not contain any free electron.
They are black coloured metallic conductors which are isomorphous with CaB6 and
contain B6 octahedral clusters . MT4B4 type of compounds ( where M = Sc , Y , Ln , Th ,
U and T = Ru , Os , Co , Rh , Ir) are of current interest due to their super conductivity .
CeCo4B4 is a well characterized compound . The reaction of metals with C or t reduction
of Ln2O3 yields carbides of stoichiometry LnC2 and Ln4(C2)3 . Carbides of lanthanoids
Sm to Lu are known . They do not contain Ln(+2) but are best described as acetylides
of Ln3+ and C22- with extra electron in a conduction band . They are more reactive than
CaC2 , show metallic conductivity and react with water giving ethyne .
2 LnC2
C2H2
+
+
6 H2O
H2
2Ln(OH)3 +
C2H4 + H2
2C2H2 + H2
C2H6
Similarly , the lanthanoids react with N , P , As , Sb and Bi to give LnN , LnP , LnAs ,
LnSb and LnBi respectively . They all crystallize in NaCl type lattice.
Table 14 : General trends in the solubility of Ln(+3) salts in water
-
Anions
-
-
-
Cl , Br , I , NO3 ,
, BrO3- , C2H3O2FOHHCO3C2O42CO32Basic NO3PO43Double MI sulphate
ClO4-
Cerium group
Yttrium group
Soluble
Soluble
Insoluble
Insoluble
Slightly soluble
Insoluble ; insoluble in
C2O42Insoluble ; insoluble in CO32Moderately soluble
Insoluble
Insoluble in M2SO4 solution
Insoluble
Insoluble
Moderately soluble
Insoluble ; soluble in C2O42Insoluble ; soluble in CO32Slightly soluble
Insoluble
Soluble in M2SO4 solution
Chemistry of Lanthanoid Ions in +4 Oxidation State
It is very rare to find lanthanoids with + 4 oxidation state , in solution . The only
lanthanoid with + 4 oxidation state which exists in solution and has any aqueous
chemistry is Ce4+ (i.e. ceric ion) . The elements Pr , Nd , Tb and Dy also form +4
states . However , these states are generally unstable in water and are found as non stoichiometric oxides and fluorides . Moreover , these species are required to be trapped
in a suitable crystal lattice for their stabilization .
Tetrapositive lanthanoid species can be obtained by carrying out oxidation with oxygen
or fluorine at elevated temperatures ( ∼ 450 – 5000C) . Ce(III) can be oxidized even at
room temperature . Cerium (+4) in solution is obtained by treatment of Ce(+3)
solutions with very powerful oxidizing agents , such as ammonium peroxodisulphate
(NH4)2S2O8 in nitric acid .
The listed electrode potential (table 8 ) describes the equilibrium when neither complexing
by the anion nor hydrolysis occurs.
Ce3+(aq)
Ce4+(aq) +
e-
In acidic medium , Ce(+4) is slightly stronger oxidizing agent than PbO2 and slightly
weaker than H2O2 . However, no potential data is available for alkaline systems. Experiments
show that Ce(+4) is a much weaker oxidizing agent in alkaline medium .
Solid ceric compounds include CeO2 , hydrous oxide CeO2. n H2O and CeF4. CeO2 is
a white solid when pure and acquires a pale – yellow colour on standing . It is obtained
by heating the cerium metal , or any of the Ce(+3) oxo – salts , in air or oxygen .
Ce +
CeO2
O2
2 Ce(OH)3 + ½ O2
Ce2(C2O4)3 +
2 O2
2 CeO2
2 CeO2
+
+
3 H2O
6 CO2
The oxidation of Ce (+3) oxide or hydroxide gives green , blue and purple coloured
products , before the final off - white to pale yellow CeO2 . The almost white
coloured CeO2 further darkens on heating. The coloured products are obtained due to
the presence of both tri - and tetravalent cerium in the same crystal (mixed - valence
compounds of a given element are often deeply coloured ) . CeO2 is an inert
material , not attacked by either strong acids or alkalis but dissolve in acids in the
presence of reducing agents (H2O2/ Sn(+2)) giving Ce(+3) solutions .
Hydrous ceric oxide CeO2.n H2O is a yellow , gelatinous precipitate obtained on heating
Ce(+4) solutions with bases . It redissolves without reduction in acids . In low acidic
medium , such solutions readily hydrolyze giving the basic salts . If the acidity is
kept sufficiently high , they can yield hydrated Ce (+4) salts . Double salts such as Ce
(NO3 )4 . 2 NH4NO3 and Ce (SO4)2 . 2 (NH4)2 SO4 can be prepared easily .
Pr forms a non - stoichiometric black oxide often formulated as Pr6O11 which can be
converted to PrO2 . Pr6O11 readily dissolves in acids to give aqueous Pr(+3) and
liberate oxygen , chlorine etc depending on the acid used . Ignition of oxo - salts of
terbium under normal conditions results in the formation of terbium (+4) oxides , with
composition ranging from TbO1.7 to TbO1.81 . Black coloured TbO can be obtained
by oxidation of Tb2O3 with air , at 4500C . All these oxides have the fluorite type structure.
Ce , Tb and Pr form tetrafluorides . Due to the high oxidizing power of fluorine , it is
able to oxidize these lanthanoids to the (+4) oxidation state also . CeF4 is comparatively
stable and crystallizes as a monohydrate . TbF4 and PrF4 are thermally unstable and are
capable of oxidizing H2O . All these tetraflourides are white solids with UF4 / ZrF4
structure , in which the metal shows dodecahedral coordination .
As mentioned earlier , except for Ce(IV) , only oxide and fluoride are known to stabilize
tetrapositive species . The compound CeF4 was prepared and characterized long ago . Later
on , TbF4 was also prepared and found to be isostructural with CeF4 . Pure PrF4 has not
been prepared , however the complex compounds Na[PrF5] and M2[PrF6] (M = Na , K ,
Rb , Cs) have been prepared and characterized . Terbium compounds such as M2[TbF6]
and dysprosium compound of the type Cs3[DyF7] have been successfully prepared and
characterized .
Complex Formation
The lanthanoids are moderate in their complex forming ability which increases with
moving to the right in the periodic table . This is in contrast with the transition metals
which form complexes readily . The difference arises may be due to larger size of the
lanthanoids and the presence of “very well - shielded” 4f orbitals which are not
available for interaction . Consequently, each lanthanoid ion effectively attracts the
ligands only by overall electrostatic forces . Hence , a decrease of complex ion
formation with a specific ligand is observed in the series Ln4+ > Ln3+ > Ln2+ .
The tendency of the anhydrous Ln3+ ions to form complexes increases from La3+ to Lu3+
but that of hydrated Ln3+ decreases in the same order . This is due to the fact that while
the size of gaseous Ln3+ ions decreases that of the corresponding hydrated ion
increases with the increase in atomic number .
Fluoro complexes are formed easily by small sized Ln3+ , but chloro complexes are not
readily formed in aqueous media or concentrated HCl . This property makes them distinct
from their counterparts , the actinoids . Lanthanoids have no or very little tendency to
form complexes with π -bonding ligands such as CO and CN- . This is again in contrast
to the transition metals which form numerous metal carbonyls and the difference is again
attributed to the unavailability of the 4f orbitals for bonding . Lanthanoids form less
stable complexes with monodentate oxygen ligands which tend to dissociate in
aqueous solution . However , in ethanolic solution , lanthanoids form triphenylarsine
oxide , [Ln(NO3)2] . (OAsPh3)4 ; pyridine - N - oxide , [Ln(pyO)8] (ClO4)3 and numerous
complexes with DMSO , of the type (DMSO)n Ln(NO3). Complexes with amines are
formed only in non - aqueous medium because water is a stronger ligand than
amine. Aryloxo complexes such as Sm(OAr)3 . (THF) and K[Ln(OAr)4], ( Ln = La , Nd , Er)
are also known. Mixed siloxo / amido complexes of the type, Y[O Si Bu Ar2] [ N(Me3Si)2]3
have also been reported. The complex, [Nd6 (OPr)17 Cl] is of special interest as it is made
up of 6 Nd atoms held together around Cl by means of bridging - OCHMe2 group .
Chelating ligands such as oxalates , citrates , tartarate , EDTA and acetylacetone form
most common and stable complexes with lanthanoids( Fig. 16 ). These complexes
water soluble and frequently have high and variable coordination numbers . Water or
solvent molecules are often attached to the central metal . Formation of such complexes is
used in ion - exchange separation. The bonding in complexes is preferentially ionic. These
chelates form highly soluble complexes in water , such that even their crystallization
may be extremely difficult . The formation of such chelated species in aqueous solution
involves stepwise equilibria of the type
Ln3+
+
AA
Ln (AA)
Ln (AA) +
AA
Ln (AA)2
Ln (AA)2
+ (n - 2)AA
Ln (AA)n
The overall equilibrium may be represented as:
Ln3+
+ n AA
Ln (AA)n
where , AA is the chelating group. and both equation and the charges of the chelated
species are neglected . The equilibrium constant for such an equilibrium is represented as
KLn (AA)n
=
[Ln (AA)n]
[Ln3+] [AA]n
Such type of constants are known as formation or stability constants . For convenience,
log10K values are used.
(a)
(b)
(c)
Fig. 16: Structures of (a) Oxalate (b) citrate and (c ) tartarate ions
Studies reveal that the most stable chelated species of the lanthanoids are those derived
from the aminepolycarboxylic acids . The anions of these acids are capable of forming
more than one chelate ring by utilizing the several oxygen and nitrogen donors that are
available (table 15) . It is apparent that an increase in the number of like chelate rings on
the lanthanoid ions , increases the value of stability constant . In general , for a given
ligand , the stability constant increases with decrease in crystal radius . This increase is
invariably in the region La3+ through Eu3+ with a break at Gd3+ as evident from fig. 17.
The gadolinium break can be associated with the corresponding break in crystal radii (fig
6) but the latter alone is insufficient to account for the former , especially for heavier
lanthanoids . Hence , for heavier lanthanoids it would be appropriate to consider the fact
that the formation constant actually measures the displacement of an equilibrium of the
type
[Ln (H2O)n ]3+
+
EDTA4 -
[Ln (EDTA ) (H2O )m]-
+
(n –m ) H2O
Thus , the effect of hydration on crystal radii , the rupture of Ln – OH2 bonds , the
formation of new bonds and the retention of water in the product are important factors
in determining the actual stability .
Table 15: Structure of some common Aminepolycarboxylic acids with possible chelate rings
Name
Formula
Nitrilotriacetic acid (H3NTA)
3
N
NHydroxyethylethylenediaminetriacetic acid (H3HEDTA)
Ethylenediaminetetracetic acid
(H4EDTA)
Possible
chelate
rings
CH2COOH
CH2COOH
CH2COOH
4
HOCH2CH2
HOOCCH2
HOOCCH2
HOOCCH2
1,2-Diaminocyclohexanetetraacetic acid (H4DCTA)
NCH2CH2N
NCH2CH2N
CH2COOH
CH2COOH
CH2COOH
5
CH2COOH
5
CH2
CH2
CH2
CH2COOH
CHN
CH2COOH
CHN
CH2
CH2COOH
CH2COOH
Fig 17 : Stability constants of lanthanoid – EDTA complexes in aqueous solutions at 20оC
The β – diketonates ( fig.18) are also commonly used as chelating ligands . These chelates
have the same colours as the aquated tripositive ions with only a slight modification in
the absorption bands. Anionic catecholate complexes with higher coordination numbers 7
and 8, such as [Gd2(cat)6]6- and [Gd(cat)4]5- are also well characterized. Various crown
ethers with differing cavity diameters and high coordination numbers are also known . For
Ln = La to Gd , the most thermally stable product has Ln : crown ether ratio as 4 :
3 , with the general formula [Ln(NO3)2 L]3 [Ln(NO3)6] . The larger lanthanoids i.e. , La
, Ce , Pr and Nd form 1:1 complexes of the type [Ln(NO3)3L] ; where L represents
the crown ether . The commonly used crown ether is 18 - crown - 6 – ether (or 1 , 4 , 7 ,
10 , 13 , 16 – hexaoxacyclo – octadecane ) (Fig. 19 ) . The coordination number of the
former complex is 12 while that of the latter is 10 . The smaller lanthanoids (Tb - Lu)
occupy the cavities in the crown ether and very large lanthanoids form [Ln(NO3)3
(H2O)3]L , in which the crown ether L remains uncoordinated . These complexes
generally dissociate instantly in water. The 8 – quinolinol chelates are uniformly yellow
but their absorption spectra consists of sharply defined , weak bands at the wavelengths
of those for the aquated ions superimposed on the broad , intense bands of the 8 –
quinolinol grouping . Different types of polyoxo ligands such as polyphosphates and
mixed phosphoryl / carbonyl species also form complexes with Ln3+ , mainly in aqueous
medium .
Fig.18: Structure of β – diketonates
R(CO)(CH-)(CO)R
Fig. 19 : Structure of 18 - crown - 6 - ether
Some super oxide complexes have also been reported . Lanthanoids form larger number of
complexes with O – donor ligands as compared to N - donor ligands . This is due to the
higher eletronegativity associated with the former type of ligands which helps in the
formation of a stable ionic bond . Conventional complexes of amines such as en , dien ,
dipy and N – bonded thiocyanato can be prepared easily in polar organic solvents . These
complexes also have high coordination numbers , usually varying from 8 to 10 . Complexes
such as [Ln(en)4]3+ , [Ln(dien)4(NO3)]2+ , [Ln(en)3Cl3] have also been reported . Sc (III) ,
Nd (III) , Eu (III) and Gd (III) form complexes with substituted amido ligands such as
( Me3Si)2 N- . Common complexes with such ligands are [(Me3Si)2N]3 Ln (THF) and
(Me3Si) [(C6H5)N]3 Nd(THF). These complexes are very stable . The lanthanoid - EDTA
complex shows no destabilization due to the presence of two N - donor atoms. Similarly ,
the N - donor analogues of crown ethers are sufficiently stable in water . Various
complexes of porphyrins , porphyrinogens and phthalocynanine are known .
Lanthanoids form complexes with sulphur as well as phosphorus ligands . Amongst the
S - ligands , dithiocarbamates and dithiophosphinates are common . Examples include
eight - coordinated [Ln(S2CNR2)4]- and [Ln(S2PMe2)4]- . Even mixed S / N - donor ligands
form stable complexes of the type {Ln[N(SiMe3)2] . [`µ-SBu/ ]}2 , where Ln = Eu , Gd
and Y . A very few phosphido complexes of the type , Ln [N(SiMe3)2]2 (PPh2) , (Ln =
La , Eu) and Tm [P(SiMe3)2]3 (THF)2 are known .
From the above discussion , it can be concluded that lanthanoids form numerous
complexes with high coordination numbers such as 7 , 8 and 9 . Lighter lanthanoids are
able to form 10 and even 12 coordinated complexes with small chelating ligands such as
NO3- and SO42. For example , [Ce(NO3)5]2- forms a 10 - coordinated bi - capped
dodecahedron , while [Ce(NO3)6]3- forms a 12 – coordinated icosahedron (fig. 20) .
Fig. 20 : Structure of [Ce(NO3)6]3—
Lanthanoids form only a few complexes with coordination number 6 which is the
most common coordination number in case of transition metal complexes ( Table 16 ) .
Table 16 : Oxidation states , coordination numbers and stereochemistries of the
compounds of lanthanoids
Oxidation
state
Coordination
number
2
6
3
8
3
4
6
7
8
9
Complex
LnZ ( Ln = Sm, Eu, Yb ;
Z = S, Se, Te)
LnF2 ( Ln = Sm, Eu, Yb
[Ln{ N(SiMe3)2}3];( Ln = Nd, Eu, Yb)
[Lu(2,6-dimethylphenyl)4][Ln{ N(SiMe3)2}3(OPPh3}] ;
(Ln = Eu, Lu)
[LnX6]3-; (X= Cl, Br), LnCl3;
( Ln = Eu, Lu)
[Ho{PhC(O)CH=C(O)Ph}3(H2O)]
[Ho(tropolonate)4][Eu(acetylacetone)3(phenanthroline)]
LnF3; ( Ln= Sm – Lu)
[Ln(H2O)9]3+ , [Eu(terpy)3]3+
[Pr(terpu)Cl3(H2O)5].3 H2O
[Ln (NO3)5]2- ; (Ln = Ce, Eu)
12
[Gd(NO3)3(DPAE)](a)
[La(acetylacetone)3(H2O)2]
[CeIV(acetylacetone)4]
[CeIV(NO3)6]3-
Stereochemistry
Octahedral
Cubic
Pyramidal
Tetrahedral
Distorted Tetrahedral
Octahedral
Capped octahedral
Dodecahedral
Square antprismatic
Bi-capped trigonal
prismatic
Tri-capped trigonal
prismatic
Capped square
antiprismatic
Bi-capped
dodecahedral
Irregular
Square antiprism
Square antiprism
Icosahedral
(each NO3- is
bidentate)
4
[CeCl6 ] 2LnO2; ( Ln = Ce, Pr, Tb)
[Ce(acac)4], LnF4; ( Ln = Ce, Pr, Tb)
6
8
[CeIV(NO3)4(Ph3PO)2]
10
[Ce(NO3)6]2-
12
Octahedral
Cubic
Square
antiprisamatic
Complex
(each NO3- is
bidentate)
Icosahedral
(each NO3- is
bidentate)
(a) DPAE = 1,2- di(pyridine-2- aldimino) ethane
However, lanthanoids can be induced to form complexes with exceptionally low
coordination numbers by using very bulky ligands such as (2 , 6 - dimethylphenyl) and
[N(SiMe3)2]- . Complexes such as [Ln{N(SiMe3)2}3] are volatile , air - sensitive , undergo
hydrolysis easily and have coordination number of 3 , the lowest possible coordination
number reported for the lanthanoids . The complex has a zero dipole moment and
pyramidal geometry in solid state which changes to planar in solution . Ph3PO forms
4 - coordinated distorted tetrahedral complex of the type [Ln{N(SiMe3)2}3 (OPPh3)] .
Six coordinated trigonal prismatic complex , Pr[S2P(C6H11)2]3 is also known . The
bonding in complexes with low coordination numbers can be explained easily as
compared to those with high coordination numbers . Even if all the valency shell
orbitals (i.e. one s , three p and five d) are used for bonding , it accounts only for a
maximum coordination number of 9 . Higher coordination numbers of 10 and 12 indicate
the participation of f - orbitals in bonding or bond orders of less than one .
Cerium is the only lanthanoid with a significant chemistry even in + 4 oxidation state .
The most important complexes formed by Ce(IV) are 12 - coordinated , water soluble
double nitrate , (NH4)2 [Ce(NO3)6] and 10 - coordinated [Ce(NO3)4 (OPPh3)2] . Complexes
with β - diketonates and other O - donor ligands are also known . The sulphates such
as , Ce(SO4)2.n H2O (n = 0 , 4 , 8 , 12) and (NH4)2 [Ce(SO4)6] are very common . Iodates
and fluoro complexes such as [CeF8]4- and [CeF6]2- are well known . [CeF6]2- is
an 8 - coordinated , square antiprismatic complex . Orange coloured octahedral chloro
complex , [CeCl6]2- is also stable .
Organometallic compounds : The organometallic chemistry of the lanthanoids is not
as extensive as that of
transition metals . This is due to the inability of
lanthanoids to engage in bonding with π - bonding ligands . However , it has shown
appreciable growth in the recent past and has become predominantly , though not
exclusively , the chemistry of yclopentadienyls , alkyls and aryls . These organometallic
compounds are thermally stable but sensitive to air and moisture .
Cyclopentadienyl Compounds : The cyclopentadienyl compounds are actually the salts
of C5H5- anion . A series of cyclopentadienyl (C5H5 or Cp) compounds of the type [Ln
(C5H5)3] , [Ln(C5H5)2Cl] and [Ln(C5H5)Cl2] are prepared in anhydrous medium by the
following reaction :
LnCl3 +
n Na( C5H5)
THF
LnCl3-n ( C5H5 ) n + n NaCl
These compounds are well characterized . The properties and structures of these
compounds depend on the nature of the solvent as well as on the size of the lanthanoid
ion and the steric demand of the cyclopentadienyl group . The structures of LnCp3
and Ln2Cp4Cl2 are given in fig 21 .
(a)
(b)
Fig. 21 : Structures of (a) [Ln(C5H5)3] and (b) [Ln2(C5H5)4Cl2]
The tris ( C5H5) compounds are insoluble in non – polar solvents whereas the bis (C5H5)
compounds dimerize . However , in polar solvents they exist as solvated monomeric
species . The heavily substituted cyclopentadienyl rings such as C5Me5 (i.e. , Cp*) make
the existence of [Ln(Cp*)3] difficult due to the steric demand . (Cp*)2LnIIIX and
SmII(Cp*)2 are very well characterized and generally serve as the centre for catalytic
polymerization of olefins . SmII(Cp*)2 undergoes dimerization with nitrogen bridging
in presence of N2 . The entire sequence of reaction is explained in fig. 22.
Fig. 22 : Formation and dimerization of SmII(Cp*)2
The structure of these cyclopentadienyl compounds is very complicated . In
[Sm(C5H5)3] , each C5H5- is pentahapto towards one metal atom but some also act as
bridges by presenting a ring vertex (η1) or edge (η 2) towards an adjacent SmIII atom .
The blue solid , [Nd(C5H4Me)3] is tetrameric , each Nd being attached to three rings in a
pentahapto manner with one ring being further attached in a monohapto manner to an
adjacent Nd . The structure of red complex , [Yb(C5H4Me)2Cl] is comparatively
simpler . It exists as a dimer with simple pentahapto rings and two chloride bridges .
Complexes with analogous ligands , C9H-7 (fig. 23 ) and C8H82- are also known .
Fig. 23 : Structure of C9H-7
For example , complexes of the type [Sm(C9H7)3 ] and K[Ln(η8-C8H8)2] are common
with lighter lanthanoids . Due to the bulky nature of the ligand , such complexes show
very little tendency to solvate . The various Ln(III) members of the series are presumed
to have “sandwich” structure. The hydrido derivatives of the Cp and Cp* lanthanoid
compounds are also known . Such compounds are highly reactive and show activity as
hydrogenation catalysts .
Simple alkyl and aryl
prepare and are rare ,
coordination numbers .
Ph3Er(THF)3 . They are
compounds of the type LnRn and LnArn are very difficult to
mainly due to the preference of large Ln(III) ions for high
LnR3 and LnAr3 compounds are usually solvated . For example
generally prepared with lithium reagents in ether solution .
LnCl3
+ 3LiR
LnR3
+ 3LiCl
LnCl3
+ 4LiR
Li[LnR4] + 3LiCl3
Bulky alkyl groups such as – CH2CMe3 and – CH2SiMe3 form stable complexes of the
type [LnR3(THF)2] . The triphenyls are usually polymeric . The first fully characterized
complex of the type was [Li(THF)4] [Lu(C6H3Me2)4] . Octahedral [LnMe6]3- have also
been isolated for most of the lanthanides . With bulky ligands , lanthanides form bis –
arene sandwich compounds (fig 24 ) .
Fig 24 : Lanthanoid Bis - Arene sandwich compounds
The divalent lanthanides , Sm , Eu and Yb form Grignard - like compounds , which are
structurally more complex than the grignards but can be effectively used as alkylating
agents .
Uses of Lanthanoids and their Compounds
(i) Uses of alloy (misch metal ) : Certain alloys of lanthanoid elements , known as
misch metals containing 50% of Ce together with smaller quantities of other light
lanthanoids are used in the production of heat resistant , stainless and instrumental steels .
These are pyrophoric alloys and when scrachted gives off sparks capable of igniting
flammable gases . Hence the alloys can be used as flints in cigarette lighters , miners
safety lamps and automatic gas lighting devices . Another rapidly expanding application
of misch metal is in batteries . Misch metal is a component of nickel metal hydride
batteries that are replacing nickel cadmium batteries in powering portable electronic
equipment such as lap tops , computers and mobile phones . Mg alloys containing about
3% misch metal and 1% Zr are used for making parts of jet engines because these
have sufficiently high strength and creep resistance at 450 - 600оF . Misch metal is an
excellent scavenger for oxygen or sulphur in many metal systems . Both misch metal and
yttrium have a marked nodulizing effect upon graphite and thus enhance the
malleability of cast iron .
(ii) Ceramic applications : These include cerium (IV) oxide and cerium – rich (> 40 %
CeO2 ) oxide mixtures as highly efficient glass – polishing compositions ; neodymium and
praseodymium oxides as colouring agents for glass and in the production of standard
filters . Lanthanum oxide is used in the preparation of low – dispersion , high – refraction
optical glass . Cerium (IV) oxide is used to improve the stability and discolouration
resistance of glass to gamma or electron – beam radiations . Cerium (IV) and neodymium
oxides are used to counteract the iron (II) – produced green in glass . 3 % praseodymium
oxide in combination with zirconium (IV) oxide is used to give a yellow ceramic glaze .
It is used to opacify enamels . The high melting points of the oxides , certain sulphides
(e.g. , CeS ) , borides , carbides , and nitrides suggest their use as refractories , although
some of these are reactive at high temperatures . Lanthanoids are also used as UV
absorbers and antibrowning agents , additives to structural ceramics such as
stabilised zirconia and silicon nitride Si3N4 , and in optical lenses and glasses . CeO2 is
used in the manufacture of protective transparent glass block . The mixture of CeO2(47%) ,
La2O3 + Nd2O3 + Pr2O3 (51%) + SiO2 , CaO , Fe2O3 is called polirite . It is used as abrasive
for polishing glasses .
(iii) Catalytic applications : Lanthanoid compounds are good catalysts for a number
of reactions such as
hydrogenation , dehydrogenation , refining of crude oil and
oxidation of various organic compounds . Addition of 1 to 5% lanthanoid chloride to
a zeolite catalyst increases the catalysts cracking efficiency . They are also included in
catalytic converters in automobiles where they stabilise the gamma - alumina support and
enhance the oxidation of pollutants. The anhydrous lanthanoid chlorides are used in
polyesterification processes ; and the chlorides and cerium phosphate in petroleum
cracking . In the sense that heterogeneous catalysts are commonly characterized by
unpaired electrons , paramagnetism , defect structures , or variability in oxidation state ,
catalysis is a promising practical area .
(iv)
Magnetic and electronic applications : Industry has been slow in taking
advantage of the para - and ferromagnetic properties of the lanthanoids . The low
electrical and eddy current losses of the ferrimagnetic garnets , 3 Ln2O3 . 5 Fe2O3 , make
these substances useful in microwave devices and as magnetic core materials . The
yttrium compound “yig” is particularly important . Certain compounds , e.g. , selenides ,
tellurides are of potential interest as semiconductors or thermoelectrics . The titanates and
stannates, are useful ceramic capacitors , because of high dielectric constants and small
temperature coefficients of capacitance .
(v) Nuclear applications : These include actual and potential uses of high – cross section metals or compounds mainly in control , shielding and flux – suppressing
devices . Their oxides are used as diluents in nuclear fuels . Metals are also used for
making structrural compounds of the reactor . For example , yttrium pipes are used which
are not attacked by molten U even at 1000оC . Tm and Eu are used as radiation
sources . Pr is used in the production of atomic batteries . Thorium is a potential
atomic fuel source .
(vi) Metallurgical applications : Lanthanoids are used as reducing agents in metallurgical
operations . About 30% of lanthanoids are used in metallurgy as an alloying agent to
desulphurise steels , as a nodularising agent in ductile iron , as lighter flints and as
alloying agents to improve the properties of superalloys and magnesium , aluminium
and titanium alloys . CeS is used in the manufacture of special crucibles which can
withstand temperatures up to 1800○C .
(vii) Miscellaneous applications : Cerium salts are used for increasing haemoglobin
content of blood and for the treatment of vomitting and sea – sickness . Lanthanoid
compounds are filled into carbon electrodes to give brilliant light . La , Ce , Eu , and
Sm salts are used in luminophores as activators and in the coatings of luminescent
lamps . Salicylates of Pr, Nd a known as dimals are used as germicides . Lanthanoid
compounds are used as insectofungicides and as trace elements in fertilizers . Ce (SO4)2
is used as an oxidizing agent in volumetric titrations . Cerium salts are used in
dyeing cotton and lead accumulators . Ce (NO3)4 is used as a mordant for alizarin dyes .
Radio isotopes like La140 , Ce142 , En152 , Tb160 are used in co – precipitation and
chromatographic separation .
The major applications of lanthanoids and yttrium , scandium and thorium are listed in
table 17 .
Table 17 : Major applications of the lanthanoids , yttrium , scandium and thorium
Element
Lanthanum
Cerium
Comment
Component of mischmetal.
Most abundant amongst the
lanthanides. Chief component of
mischmetal.
Praseodymium
Neodymium
Important in magnetic alloys.
Applications
Ceramic glazes, high quality
optical glass, camera lenses,
microwave crystals, ceramic
capacitors, glass polishing,
petroleum cracking.
Glass polishing, petroleum
cracking catalysts, alloys - with
iron for sparking flints for lighters,
with aluminium, magnesium and
steel for improving heat and
strength properties, radiation
shielding, many others.
Yellow ceramic pigments, tiles,
ceramic capacitors. With
neodymium in combination for
goggles to shield glass makers
against sodium glare, Permanent
magnets. Cryogenic refrigerant.
Ceramic capacitors, glazes and
coloured glass, lasers, high strength
permanent magnets as neodymiumiron-boron alloy, petroleum
cracking catalysts.
Promethium
Not found in nature. Radioactive;
produced only in nuclear reactors.
Samarium
Important in magnetic alloys.
Europium
One of rarest, and most rare
reactive of rare earths. Absorbs
neutrons.
Gadolinium
Terbium
Associated with gadolinium.
Dysprosium
Absorbs neutrons. Magnetic alloy.
Holmium
Absorbs neutrons.
Erbium
Physical properties almost identical
with Holmium and Dysprosium.
Gives x-rays on irradiation in
nuclear reactor.
Thulium
Ytterbium
Lutetium
Properties very similar to Lutetium
- not well known.
Chemical and physical properties
not well known.
Yttrium
Associated with Holmium, Erbium.
Cold or hot forged
Scandium
Close to Aluminium in chemical
and physical properties
Thorium
Resembles nickel, as soft and as
plentiful as lead. Radioactive.
Radioactive promethium in
batteries to power watches, guided
missile instruments, etc, in harsh
environments.
In highly magnetic alloys for
permanent magnet as SamariumCobalt alloy; probably will be
superseded by neodymium. Glass
lasers. Reactor control and neutron
shielding.
Control rods in nuclear reactors.
Coloured lamps, cathode ray tubes.
Red phosphor in colour television
tubes.
Solid state lasers, constituent of
computer memory chips, high
temperature refractories, cryogenic
refrigerants.
Cathode ray tubes, magnets, optical
computer memories; future hard
disk components; magnetostrictive
alloys.
Controls nuclear reactors. Alloyed
with neodymium for permanent
magnets. Catalysts.
Controls nuclear reactors; catalysts;
refractories.
In ceramics to produce a pink
glaze; infra-red absorbing glasses.
X-ray source in portable X-ray
machines.
Practical values presently
unknown. Research.
Deoxidiser in stainless steel
production, rechargeable batteries,
medical uses, red phosphors for
colour television, superconductors.
Deoxidiser in stainless steel
production, rechargeable batteries,
medical uses, red phosphors for
colour television, superconductors.
X-ray tubes, catalysts for
polymerisation, hardened Ni-Cr
superalloys, dental porcelain.
Gas mantles. Can be used as
nuclear fuel in place of uranium.
Some Comparisons and Contrasts
As mentioned in the beginning , both yttrium and scandium could be considered with
lanthanoids because of the similarities in their physical and chemical properties .
Yttrium is similar to the lanthanoids in following respects :
1] Y occurs with lanthanoids in rare earth minerals , eg. monazite ;
2] The radius of Y3+ is comparable to that of Ho3+ and hence it is difficult to separate
Y3+ from Ho3+ ;
3] Y occurs effectively exclusively in + 3 oxidation state .
4] YH2 is better known to exist as Y3+(H-)2(e-) , similar to Ln3+(H-)2(e-) .
Likewise , scandium is similar to the lanthanoids , as detailed below :
1] Sc occurs effectively exclusively in +3 oxidation state . Compounds like Sc2O3 , ScX3
are well known ;
2] Sc forms conducting hydride ScH2 , better known as Sc3+(H-)2(e-) ;
3] Sc forms reduced halides of the type Sc7Cl12 , which is Sc3+ with Sc6 clusters .
4] Sc forms complexes of high coordination number with chelating O – donors e.g.
Na+[Sc(CF3COCHCOCF3)]4 with coordination number 8 .
5]
Most of the scandium compounds are obtained as hydrated salts e.g.
Sc(NO3)3.4H2O and Sc2(SO4)3.5H2O.
6]
Scandium forms organometallic compounds such as Sc(C5H5)3 .
However , there are sufficient reasons to consider scandium as a 13 group element , with
aluminium . In fact , it can be considered to be intermediate between the lanthanoids and
aluminium .
1]
Sc3+ (r = 74 pm) is appreciably smaller than any of the Ln3+ .
2]
Sc2 O3 is amphoteric like Al2O3 and not like Ln2O3 .
3]
ScF3 dissolves in excess F- to give ScF63 - . Lanthanoids , on the contrary , show
scarcity of halogen compounds .
4]
Anhydrous ScCl3 is easily obtained by dehydration of respective hydrated halide
with P2O5 . However , unlike AlCl3 , ScCl3 is not a Friedal - Crafts catalyst .
In some of the properties , scandium even resembles the first row transition metals . Like
3d metals , Sc usually forms complexes with coordination number 6 and its aqua ion ,
[Sc(H2O)6 ]3+ is susceptible to hydrolysis .
As discussed earlier , the lanthanoids show some similarities and some dissimilarities
with transition elements as well as actinoides . Some contrasts between lanthanoids ,
pre – transition and transition metals are given in table 18. Comparison between
lanthanoids and actinoids are listed in table 19 .
Table 18: Some contrasts between Lanthanoids , Pre – Transition & Transition Metals
Property
Pre-Transition Metals
Lanthanoids
Transition Metals
Valence
Essentially Monovalent
Show Group(n+) oxidation
state
Periodic trends
Dominated by effective
nuclear charge
Widespread on earth
No effects
Always ‘hard’ (prefer O, Hl,
N donors)
Essentially Monovalent(+3) is
characteristic oxidation state
+2/+4 for certain
configurations
Dominated by Lanthanide
Contraction of Ln3+
Common mineralogy
Insignificant effects
Always ‘hard’ (prefer O, Hal,
N donors)
Show Variable Valence
(extensive redox chemistry)
control by environment ∼
ligands, pH etc…
Size changes are less
marked
Diverse mineralogy
Substantial effects
Later(increasingly from
Fe)/heavier metals may
show a ‘soft’ side
‘Covalent’ Organometallics
Occurence
Ligand field effect
Nature
Nature of
organometallics
Coordination
number
‘Ionic & ‘Covalent’
Organometallics
Poor Coordination Properties
(C.N. determined by size)
‘Ionic’ organometallics
Geometry
Flexibility in Geometry
Flexibilty in Geometry
Magnetism
No Magnetism from the
metal ions due to noble gas
configurations of ions
Free Ion ground state
magnetism
Spectra/ Properties
‘Ionic’ compound
formulations
large HOMO-LUMO gaps
UV spectra
Weak, Narrow Optical Spectra
Forbidden, unfacilitated
transitions
High Coordination Numbers –
6,7,8,9 (C.N. determined by
size)
Extensive Coordination
C.N. = 4 and 6 are typical
maximum (but many
exceptions)
Fixed(by Ligand Field
effects)Geometries
Orbital Magnetism
‘Quenched’ by Ligand
Fields
Excited J-states poulated
Stronger, Broader Optical
Spectra
Forbidden transitions
vibronically-assisted
Table 19 : Comparison between lanthanoids and actinoids
Property
Lanthanoids
Actinoids
Valence shell configuration
Characteristic oxidation state
Filling of the 4f subshell
+3, +2 and +4 are known but rare
Contraction
Lanthanoid curve consists of two very
shallow arcs with a discontinuity at a
spherically Gd3+
4f orbital are sufficiently low in
energy that the e-s are seldom ionized
or shared. They are unaffected by the
environment to a great extent
Less
Spectra are sharp and line-like
5f subshell
Although all the actinoids exhibit a +3
oxidation state, it is not the most stable
one for most of them. Oxidation state
as high as +7 are also achieved,
especially in early actinoids
Actinoid contraction initially parallels
that of lanthanides, the elements from
curium are smaller than expected
5f orbitals have a radial node, which 4f
orbital lack. 5f electrons are available
in the early actinides
Nature of f orbital
Ligand-metal interaction
Absorption spectra
Magnetic properties
Coordination chemistry
Spin-orbit coupling is large but
ligand field effects are small.
Show high coordination
More
Am3+ and heavier actinides have
spectra resembling Ln3+ but P43+ and
lighter actinoids have broader spectra
Spin-orbit coupling and ligand field
effects are of comparable magnitude
Show high coordination