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DOI: 10.1595/147106708X297486
The Periodic Table and the Platinum
Group Metals
By W. P. Griffith
Department of Chemistry, Imperial College, London SW7 2AZ, U.K.; E-mail: [email protected]
The year 2007 marked the centenary of the death of Dmitri Mendeleev (1834–1907). This
article discusses how he and some of his predecessors accommodated the platinum group
metals (pgms) in the Periodic Table, and it considers the placing of their three transuranic
congeners: hassium (108Hs), meitnerium (109Mt) and darmstadtium (110Ds). Over twenty-five
years ago McDonald and Hunt (1) wrote an excellent account of the pgms in their periodic
context. This account is indebted to that work. The present article introduces new perspectives
and shows some of the relevant tables. There are good books on the history of the Periodic
Table, e.g. (2, 3) and other texts (4, 5) which provide a fuller picture than it is possible to
give here.
Discovery and Early Classification
of the Platinum Group Metals
Antoine-Laurent Lavoisier (1743–1794) in 1789
defined the element as being “the last point that
analysis can reach”, and it was largely this clear statement which brought about the discovery of 51 new
elements in the nineteenth century alone. John
Dalton’s (1766–1844) recognition in 1803 of the
atom as being the ultimate constituent of an element,
with its own unique weight, was crucial. Stanislao
Cannizzaro (1826–1910), at the celebrated Karlsruhe
Congress (1860), published a paper recognising the
true significance of Avogadro’s molecular hypothesis
and thereby clarified the difference between atomic
and molecular weights. From then, reasonably accurate atomic weights of known elements became
readily available and greatly helped the construction
of useful Periodic Tables. Atomic (or elemental)
weights were useful but were not a sine qua non for
table construction. A number of tables were produced with incorrect values, or, as Mendeleev later
noted, inconsistencies in published atomic weights
became apparent from these tables. We have the
benefit of hindsight and know that atomic numbers
are crucial factors for periodicity.
Platinum is a metal of antiquity, but the other five
pgms were isolated in the nineteenth century. The
bicentenaries of four were marked in this Journal:
William Hyde Wollaston’s (1766–1828) discovery of
Platinum Metals Rev., 2008, 52, (2), 114–119
palladium and rhodium in 1802 and 1804 (6) and
Smithson Tennant’s (1761–1815) isolation of iridium and osmium in 1804 (7, 8). Ruthenium was the
last to be isolated, by Karl Karlovich Klaus
(1796–1864) in 1844 (9–11). Thus, five of the six
were known by 1804, and the sixth by 1844, in good
time for the development of the Periodic Table.
The pgms are now known to fall into two horizontal groups: Ru-Rh-Pd and Os-Ir-Pt, but we
benefit from some 200 years of hindsight in this
observation. Johann Döbereiner (1780–1849) noted
similarities in the chemical behaviour of ‘triads’ of
elements, in which the equivalent weight of the middle element lay roughly halfway between those of the
other two. In 1829, when Professor of Chemistry at
Jena, he used his equivalent weights for these metals
(based on oxygen = 100) to demonstrate that Pt-IrOs and Pd-‘pluran’-Rh ‘triads’ existed (12). ‘Pluran’
had been reported together with two other ‘new’ elements in 1827 by Gottfried Osann (1796–1866). It
may possibly have contained some ruthenium, but
Berzelius was unable to confirm the novelty of these
three elements, and Osannn subsequently withdrew
his claim (13).
In 1853 John Hall Gladstone (1827–1902), then
a chemist at St. Thomas’s Hospital, London, noted
that the Rh-Ru-Pd triad was related to that of
Pt-Ir-Os, while the ‘atomic weights’ (sic) of the latter
triad were roughly twice those of the former (14). In
114
1857 William Odling (1829–1921), then teaching
chemistry at Guy’s Hospital, London, noted the
great similarity of Pd, Pt and Ru, that the ‘atomic
weight’ (sic) of Pt (98.6) was about twice that of Pd
(53.2), and that Pt, Ir and Os were chemically similar
(15). The stage was now set for a periodic classification of these and indeed all the elements then
known.
The Development of Periodic
Classifications
In 1862 Alexandre-Emile Béguyer de
Chancourtois (1820–1886), Professor at the École
des Mines, Paris, devised a ‘vis tellurique’ (telluric
screw) (16), a helix on a vertical cylinder on which
symbols of the elements were placed at heights proportional to their atomic weights. Although some
pgms appeared on it (Rh and Pd on one incline and
Ir and Pt on another), no relationships between
them are discernible.
Karl Karlovich Klaus, then professor of chemistry at the University of Kazan (now in Tatarstan),
had discovered Ru in 1844 (9–11) and knew more
about the pgms than anyone else. In 1860
he arranged the three most abundant ones in a
Principal series (Haupt Reihe), and beneath them
placed a Secondary series (Neben Reihe), noting
also the chemical similarities of each vertical pair
(17–19) (Figure 1 (18)).
Klaus’s table shows the correct vertical pairs, but
not in the now accepted sequence. The pgms were
not set in the context of other elements. In 1864 the
analytical chemist John Alexander Raina Newlands
(1837–1898) proposed the first of his tables, arranging the known 61 elements in order of ascending
atomic weights (20, 21). In his subsequent ‘law of
octaves’ he noted that the chemical properties of
some elements were repeated after each series of
seven, and assigned ordinal numbers to elements in
the sequence of their ascending atomic weights: an
early form of the atomic number (e.g. H = 1, Li = 2
etc.) (22). Although the pgms featured in Newlands’s
tables they were often out of place. William Odling
(born, like Newlands, in Southwark, London),
whose pgm triads we have noted above (15), produced in 1864 a table of 61 elements in which the six
pgms were grouped together (Ro is rhodium). He
was the first to arrange them in a reasonably logical
way in a Periodic Table (Figure 2) (23).
The stage was now set for two giants of periodicity, Lothar Meyer and, above all, Dmitri Mendeleev.
In 1868 Julius Lothar Meyer (1830–1895), Professor
of Chemistry at Tübingen arranged 52 elements in
an unpublished table with Ru & Pt, Rh & Ir, Pd &
Os side-by-side. His slightly later table, published in
1870 (24), places the pgms correctly, but a number
of other elements lie in a sequence different from
that of modern tables:
Mn = 54.8
Ru = 103.5
Os = 198.6?
Fe = 55.9
Rh = 104.1
Ir = 196.7
Co = Ni = 58.6 Pd = 106.2
Pt = 196.7
On 6th March, 1869, Dmitri Mendeleev
(1834–1907) produced his first table (25, 26).
Mendeleev was born in Tobolsk, Siberia, the last of
fourteen children. His father became blind when
Dmitri was sixteen, and his indomitable mother,
determined that he should be well educated, hitchhiked with him on the 1400 mile journey to the
University at Moscow. Here he was refused admittance because he was Siberian; they travelled a
further 400 miles to St. Petersburg. There in 1850
Mendeleev got a job as a trainee teacher; his mother
died from exhaustion in the same year. In 1866, after
a spell of study in Germany (he had attended the
1860 Karlsruhe Congress) and France, Mendeleev
became Professor of Chemistry at the University of
St. Petersburg.
Mendeleev’s interest in periodicity may well have
dated from the Karlsruhe Congress and been
Fig. 1 Klaus’s arrangement of the
platinum group metals of 1864 (18)
Platinum Metals Rev., 2008, 52, (2)
115
Fig. 2 William Odling’s table of
elements from 1864 (23)
cemented by a textbook on inorganic chemistry, part
of which he finished in 1868. More than any of his
predecessors in the field of periodicity, he had a
remarkable knowledge of the chemistry of the elements. His first published version placed the pgms
together but with unusual pairings (25, 26):
Rh 104.4
Pt 197.4
Ru 104.4
Ir 198
Pd 106.6
Os 199
The version normally regarded as Mendeleev’s
definitive table appeared in 1871, first printed in a
Russian journal (27) and then reprinted in Annalen in
the same year (Figure 3) (28). By then Mendeleev
had seen Lothar Meyer’s paper and almost certainly
knew of Newlands’s and Odling’s work, but his table
represents a major advance in classification of the
elements, for the first time placing the pgms in their
modern sequence and in context. The dashes under
the Ru-Rh-Pd-Ag listing under Group VIII misled
some later workers to think that missing elements
were being denoted (13). Acceptance of his table was
Platinum Metals Rev., 2008, 52, (2)
partly brought about by his astonishingly accurate
predictions of the properties of the then unknown
scandium (shown as ‘–- = 44’ in Figure 3), gallium
‘–- = 68’ and germanium ‘–- = 72’. Mendeleev’s predictions also led to the subsequent discovery of other
elements including francium, radium, technetium,
rhenium and polonium. Other factors such as the
successful accommodation or placement of the elements were also important, a topic well discussed in
a recent book (3).
It is apparent from Mendeleev’s tables that for
him (and others) the pgms, some of the transition
metals, lanthanides and actinides then known posed
a problem; here we concentrate on the pgms. He
noted their very similar properties and that there
were very small differences between the atomic
weights of Ru-Rh-Pd and between those of Os-IrPt (28). He knew that only Ru and Os demonstrated
octavalency in Group VIII (‘RO4’; R denotes an element), but includes Rh, Pd, Ir and Pt in Group
VIII. Mendeleev also placed iron, cobalt and nickel,
and the coinage metals copper, silver and gold in
116
Fig. 3 Mendeleev's Periodic Table of 1871 (28)
Group VIII; he additionally accommodated the
coinage metals in Group I. His problems with all his
Group VIII elements continued to trouble him: as
late as 1879 he published two papers in Chemical
News which tried to address this difficulty (29, 30).
In the first paper he split Groups I–VII into lefthand ‘even’ and right-hand ‘odd’ blocks, with
Group VIII in the centre, Cu, Ag and Au being
accommodated in both VIII and the ‘odd’ I–VII
block (29). In the second paper he ruefully refers to
Group VIII as ‘special’ and ‘independent’ (30).
Mendeleev published some thirty Periodic
Tables and left another thirty unpublished (3), but
the 1871 one (Figure 3) (28) is his most successful:
it is the definitive Periodic Table of the nineteenth
century and the basis of all later ones. As late as
1988, the leading inorganic textbook “Advanced
Inorganic Chemistry”, by Cotton and Wilkinson
(fifth edition) (31) shows Group VIII as containing
the nine elements Fe, Co, Ni and the pgms (Cu, Ag
and Au are designated as Group IB). It was only in
the sixth edition of 1999 that the modern form
(Figure 4), in which the pgm vertical pairs are in
Groups 8, 9 and 10, was used (32).
The Transuranic Congeners of the
Platinum Group Metals
The story now moves forward to the Second
World War, when there was discussion as to
whether uranium, neptunium and plutonium were
Platinum Metals Rev., 2008, 52, (2)
appropriately placed in the fourth row of the transition metals (using 6d orbitals), or were members
of a lanthanide-like series, the ‘actinides’, using 5f
orbitals. The latter view prevailed (33), and now all
the actinides (thorium to lawrencium inclusive) are
known. Indeed, elements up to and including 118
are now established, with the exception of element
117 (34). These elements are recognised by the
International Union of Pure and Applied
Chemistry (IUPAC), although only those up to 111
have ‘official’ names (Figure 4) (35); see also (36).
Mendeleev’s table (28) omits most of the lanthanides and actinides and, of course, the noble
gases which were not known when he made up his
table. However, some 140 years earlier, his version
had essentially contained the kernel of our modern
Periodic Tables.
Recent chemical work on a few very short-lived
atoms of each element strongly suggests that elements 104 to 111 are members of a fourth
transition metal series involving 6d orbitals. Thus
104
rutherfordium, 105dubnium, 106seaborgium and
107
bohrium have properties analogous to those of
hafnium (Group 4), tantalum (Group 5), tungsten
(Group 6) and rhenium (Group 7) respectively.
The next three elements were all made in the
linear accelerator in the city of Darmstadt, Hessen,
Germany. Hassium was first made in 1984, and
named from the Latin ‘Hassias’ for the state of
Hessen. Meitnerium was first made in 1982, and
117
Fig. 4 The current Periodic Table (35) based on IUPAC recommendations
named after Lise Meitner (1878–1968), the discoverer of protactinium in 1917. Darmstadtium
was first made in 1994, and named after
Darmstadt. For any meaningful chemistry to be
carried out on a given element, at least four atoms
are necessary, of half-life (t½) > 1 second, and a
production rate of at least one atom per week is
required. The nuclear reactions producing the elements should give only single products. For these
three elements the most useful nuclear reactions
are (Equations (i)–(iii)):
269, 270
1
Cm + 26
108Hs + 5 or 4 0 n
12Mg →
248
96
Bi + Fe →
209
83
58
26
266
109
1
0
Mt + n
271
1
Pb + 64
28Ni → 110 Ds + 0 n
208
82
(i)
(ii)
(iii)
Of these, 269Hs and 270Hs have t½ = 14 and 23 s
respectively; 266Mt has t½ = 6 × 10–3 s and 271Dt has
t½ = 6 × 10–2 s, so at present chemistry can only be
carried out on hassium. It is clearly a congener of
Os: using just seven atoms it was found to form a
volatile tetroxide (37) which in alkaline NaOH
gives a species which is probably cisNa2[HsO4(OH)2] (38). For studies on meitnerium
and darmstadtium to be made, longer-lived isotopes are essential – they would also be much
Platinum Metals Rev., 2008, 52, (2)
more difficult to study chemically, since distinctive volatile Ir and Pt compounds are rare and
difficult to synthesise on a very small scale, unlike
HsO4, although the fluorides IrF6 and PtF6 are
volatile above 60ºC. It seems likely, however, that
these three elements are congeners of Os, Ir and
Pt, particularly since it has recently been shown
that the unnamed (at the time of writing) element
112 is itself volatile. This suggests that it is a congener of mercury (39), as would be expected if
elements 104–111 inclusive form a fourth transition metal series.
Conclusions
The story of the Periodic Table is convoluted,
and this article has concentrated on the pgms. It
is clear that they represented a challenge to the
makers of the tables, but the problem was finally
resolved by Mendeleev some 140 years ago (28).
The three man-made congeners of these elements, hassium, meitnerium and darmstadtium,
are likely to have chemistries similar to those of
osmium, iridium and platinum. At the time of
writing it has been possible to demonstrate this
only for hassium.
118
Acknowledgements
I am grateful to Professor Christoph Düllmann
(Gesellschaft für Schwerionenforschung mbH,
Darmstadt, Germany) and Dr Simon Cotton
(Uppingham School, Rutland, U.K.) for their advice
on aspects of transuranium chemistry.
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The Author
Bill Griffith is an Emeritus Professor of Chemistry at Imperial College, London. He has much experience with the platinum
group metals, particularly ruthenium and osmium. He has published over 260 research papers, many describing complexes
of these metals as catalysts for specific organic oxidations. He has written seven books on the platinum metals, and is
currently writing another on oxidation catalysis by ruthenium complexes. He is the Secretary of the Historical Group of the
Royal Society of Chemistry.
Platinum Metals Rev., 2008, 52, (2)
119