The Periodic Table and the Platinum Group Metals
The Periodic Table and the Platinum Group Metals
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 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-Ir-Os 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 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, hitch-hiked 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 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 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-Ir-Pt (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 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 left-hand ‘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 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 104rutherfordium, 105dubnium, 106seaborgium and 107bohrium 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 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), (ii) and (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 cis-Na2[HsO4(OH)2] (38). For studies on meitnerium and darmstadtium to be made, longer-lived isotopes are essential – they would also be much 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 60°C. 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.
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.
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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.