The Periodic Table and the Platinum Group Metals
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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 palladium and rhodium in 1802 and 1804 (6) and of the Platinum Group Metals Smithson Tennant’s (1761–1815) isolation of iridi- Antoine-Laurent Lavoisier (1743–1794) in 1789 um and osmium in 1804 (7, 8). Ruthenium was the defined the element as being “the last point that last to be isolated, by Karl Karlovich Klaus analysis can reach”, and it was largely this clear state- (1796–1864) in 1844 (9–11). Thus, five of the six ment which brought about the discovery of 51 new were known by 1804, and the sixth by 1844, in good elements in the nineteenth century alone. John time for the development of the Periodic Table. Dalton’s (1766–1844) recognition in 1803 of the The pgms are now known to fall into two hori- atom as being the ultimate constituent of an element, zontal groups: Ru-Rh-Pd and Os-Ir-Pt, but we with its own unique weight, was crucial. Stanislao benefit from some 200 years of hindsight in this Cannizzaro (1826–1910), at the celebrated Karlsruhe observation. Johann Döbereiner (1780–1849) noted Congress (1860), published a paper recognising the similarities in the chemical behaviour of ‘triads’ of true significance of Avogadro’s molecular hypothesis elements, in which the equivalent weight of the mid- and thereby clarified the difference between atomic dle element lay roughly halfway between those of the and molecular weights. From then, reasonably accu- other two. In 1829, when Professor of Chemistry at rate atomic weights of known elements became Jena, he used his equivalent weights for these metals readily available and greatly helped the construction (based on oxygen = 100) to demonstrate that Pt-Ir- of useful Periodic Tables. Atomic (or elemental) Os and Pd-‘pluran’-Rh ‘triads’ existed (12). ‘Pluran’ weights were useful but were not a sine qua non for had been reported together with two other ‘new’ ele- table construction. A number of tables were pro- ments in 1827 by Gottfried Osann (1796–1866). It duced with incorrect values, or, as Mendeleev later may possibly have contained some ruthenium, but noted, inconsistencies in published atomic weights Berzelius was unable to confirm the novelty of these became apparent from these tables. We have the three elements, and Osannn subsequently withdrew benefit of hindsight and know that atomic numbers his claim (13). are crucial factors for periodicity. In 1853 John Hall Gladstone (1827–1902), then Platinum is a metal of antiquity, but the other five a chemist at St. Thomas’s Hospital, London, noted pgms were isolated in the nineteenth century. The that the Rh-Ru-Pd triad was related to that of bicentenaries of four were marked in this Journal: Pt-Ir-Os, while the ‘atomic weights’ (sic) of the latter William Hyde Wollaston’s (1766–1828) discovery of triad were roughly twice those of the former (14). In Platinum Metals Rev., 2008, 52, (2), 114–119 114 1857 William Odling (1829–1921), then teaching early form of the atomic number (e.g. H = 1, Li = 2 chemistry at Guy’s Hospital, London, noted the etc.) (22). Although the pgms featured in Newlands’s great similarity of Pd, Pt and Ru, that the ‘atomic tables they were often out of place. William Odling weight’ (sic) of Pt (98.6) was about twice that of Pd (born, like Newlands, in Southwark, London), (53.2), and that Pt, Ir and Os were chemically similar whose pgm triads we have noted above (15), pro- (15). The stage was now set for a periodic classifica- duced in 1864 a table of 61 elements in which the six tion of these and indeed all the elements then pgms were grouped together (Ro is rhodium). He known. was the first to arrange them in a reasonably logical way in a Periodic Table (Figure 2) (23). The Development of Periodic The stage was now set for two giants of periodic- Classifications ity, Lothar Meyer and, above all, Dmitri Mendeleev. In 1862 Alexandre-Emile Béguyer de In 1868 Julius Lothar Meyer (1830–1895), Professor Chancourtois (1820–1886), Professor at the École of Chemistry at Tübingen arranged 52 elements in des Mines, Paris, devised a ‘vis tellurique’ (telluric an unpublished table with Ru & Pt, Rh & Ir, Pd & screw) (16), a helix on a vertical cylinder on which Os side-by-side. His slightly later table, published in symbols of the elements were placed at heights pro- 1870 (24), places the pgms correctly, but a number portional to their atomic weights. Although some of other elements lie in a sequence different from pgms appeared on it (Rh and Pd on one incline and that of modern tables: Ir and Pt on another), no relationships between Mn = 54.8 Ru = 103.5 Os = 198.6? them are discernible. Fe = 55.9 Rh = 104.1 Ir = 196.7 Karl Karlovich Klaus, then professor of chem- Co = Ni = 58.6 Pd = 106.2 Pt = 196.7 istry at the University of Kazan (now in Tatarstan), had discovered Ru in 1844 (9–11) and knew more On 6th March, 1869, Dmitri Mendeleev about the pgms than anyone else. In 1860 (1834–1907) produced his first table (25, 26). he arranged the three most abundant ones in a Mendeleev was born in Tobolsk, Siberia, the last of Principal series (Haupt Reihe), and beneath them fourteen children. His father became blind when placed a Secondary series (Neben Reihe), noting Dmitri was sixteen, and his indomitable mother, also the chemical similarities of each vertical pair determined that he should be well educated, hitch- (17–19) (Figure 1 (18)). hiked with him on the 1400 mile journey to the Klaus’s table shows the correct vertical pairs, but University at Moscow. Here he was refused admit- not in the now accepted sequence. The pgms were tance because he was Siberian; they travelled a not set in the context of other elements. In 1864 the further 400 miles to St. Petersburg. There in 1850 analytical chemist John Alexander Raina Newlands Mendeleev got a job as a trainee teacher; his mother (1837–1898) proposed the first of his tables, arrang- died from exhaustion in the same year. In 1866, after ing the known 61 elements in order of ascending a spell of study in Germany (he had attended the atomic weights (20, 21). In his subsequent ‘law of 1860 Karlsruhe Congress) and France, Mendeleev octaves’ he noted that the chemical properties of became Professor of Chemistry at the University of some elements were repeated after each series of St. Petersburg. seven, and assigned ordinal numbers to elements in Mendeleev’s interest in periodicity may well have the sequence of their ascending atomic weights: an 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 partly brought about by his astonishingly accurate of which he finished in 1868. More than any of his predictions of the properties of the then unknown predecessors in the field of periodicity, he had a scandium (shown as ‘–- = 44’ in Figure 3), gallium remarkable knowledge of the chemistry of the ele- ‘–- = 68’ and germanium ‘–- = 72’. Mendeleev’s pre- ments. His first published version placed the pgms dictions also led to the subsequent discovery of other together but with unusual pairings (25, 26): elements including francium, radium, technetium, Rh 104.4 Pt 197.4 rhenium and polonium. Other factors such as the Ru 104.4 Ir 198 successful accommodation or placement of the ele- ments were also important, a topic well discussed in Pd 106.6 Os 199 a recent book (3). The version normally regarded as Mendeleev’s It is apparent from Mendeleev’s tables that for definitive table appeared in 1871, first printed in a him (and others) the pgms, some of the transition Russian journal (27) and then reprinted in Annalen in metals, lanthanides and actinides then known posed the same year (Figure 3) (28). By then Mendeleev a problem; here we concentrate on the pgms. He had seen Lothar Meyer’s paper and almost certainly noted their very similar properties and that there knew of Newlands’s and Odling’s work, but his table were very small differences between the atomic represents a major advance in classification of the weights of Ru-Rh-Pd and between those of Os-Ir- elements, for the first time placing the pgms in their Pt (28). He knew that only Ru and Os demonstrated modern sequence and in context. The dashes under octavalency in Group VIII (‘RO4’; R denotes an ele- the Ru-Rh-Pd-Ag listing under Group VIII misled ment), but includes Rh, Pd, Ir and Pt in Group some later workers to think that missing elements VIII.