Platinum Metals Rev., 1985, 29, (3), 126
A Half Century of Platinum Metal Chemistry
Reminiscences of Progress from Academic Backwater to Industrial Importance
It was about fifty years ago that I first developed a serious interest in platinum metal chemistry from my research supervisor Dr. F. G. Mann (1) in the University Chemistry Laboratory in Pembroke Street, Cambridge. It was then quite a different world: each student bench had its own individual gas light and you can imagine the fire hazard. (We students joked about our Professor, Sir William Pope, having shares in the gas company!) Even to get £10 for a piece of equipment was a feat which I achieved only once. Students bought their own research chemicals and paid for their breakages. I was lucky as Dr. Mann allowed me to draw on his personal account for chemicals, but even then we made n-propyl bromide and such from the cheaper alcohols rather than buy the alkyl halide. Thus I found myself engaged in a very productive investigation of halogen bridged palladium complexes. We must remember that in the early 1930s many chemists did not believe that monovalent halogens such as chlorine could bind covalently two metal atoms although the idea was at least ten years old (2). Halogen bridging was only finally established in 1937 by an X-ray structure of the dimeric “diethylmonobromogold” (3).
Complex Chemistry in the Doldrums
The general character of platinum metal chemistry was well established before 1935. The main point at issue had been the nature of the isomerism of its various compounds, especially of the α- and β -isomers of the platinum(II) complexes. With the advent of Werner’s famous theory (4) the planar configuration had slowly, but eventually, been accepted by most chemists. The isomeric ammines had been oriented by him using methods closely akin to the classical methods used to orient the products of aromatic substitution. The deeper coloured α -[PtCl2(NH3)2] was correctly assigned a cis- and the paler β -isomer a trans -configuration, but the remaining such isomers, for example of the tertiary organic phosphines and sulphides, had been assigned incorrect configurations by analogy of colour. So they remained until 1936 when K. A. Jensen in Copenhagen, by measuring the dipole moments of platinum(II) complexes soluble in organic solvents, showed that the paler coloured isomers, except the ammines, were the cis -isomers (5). It was also at about this time that X-ray structure determination came to the aid of co-ordination chemists, although not entirely reliably.
This advance in structural technique is important because acceptance of Werner’s ideas had been so slow that still they had not entered the main stream of chemistry. Even in the early 1930s Professor H. D. K. Drew (6) whom I remember well, was trying to show that the isomeric platinum(II) complexes were structural rather than geometrical isomers, according to the ideas of Blomstrand (7) and Jörgensen (8) from the last century. They had eventually been discredited by Werner after long controversy with Jörgensen, but not completely laid to rest. Drew produced evidence which he claimed to show that α -[PtCl2(SEt2)2], now known as the trans -isomer, had a tetrahedral structure and that the β -isomer was a disulphonium salt of structure Cl · · · Et2S—Pt—SEt2 · · · Cl where · · · represents an electrovalency. Even such nonsensical structures as (I) for “ethylene platinous chloride” (9) and (II) for “nickel carbonyl” (10) were appearing in the 1930s and even the 1940s.
That such structures could appear at the time without furore in chemical circles shows how little co-ordination chemistry and Werner’s theory were appreciated by the general body of chemists. “British Abstracts” had condensed Werner’s famous 68 page paper (4) expounding his Nobel Prize winning theory to eight lines of trivial abstract (11); clearly the abstractor did not understand it at all. The olefin complexes were still regarded as chemical curiosities of little significance. Around 1950 when I was making my study of platinum(II) olefin complexes which led to the proposal, with supporting evidence, of what is now the generally accepted structure, I was visited by a senior academic who later became a Nobel Laureate. After I had discussed my work with him he finally commented “Nice little field you have carved out for yourself, Chatt, but…” and then in the nicest possible way indicated that I had entered a backwater of chemistry of little interest to anyone. The paper (12) is now a citation classic, counting only citations since 1965 (13).
Even after the Second World War there had been no conference on any aspect of transition metal chemistry so long as I could remember. My complaint about this to the Acting Manager Mr. M. T. Sampson, of the Butterwick Research Laboratories of Imperial Chemical Industries Limited where I was employed led to the first of the now well known series of International Conferences on Coordination Chemistry (I.C.C.C.). It was held on 21st-22nd September 1950 in the Butterwick Laboratories, financed by our company, and organised privately by myself and Dr. R. G. Wilkins. We invited everyone in the U.K. who had published anything on co-ordination chemistry since 1930: only one, who was ill, did not come. We collected 31 participants from outside I.C.I. and two from abroad, K. A. Jensen and G. Schwarzenbach. The size of the meeting reflects the abysmal interest in complex chemistry in the U.K. at that time, rather than the poor life expectancy of co-ordination chemists. K. A. Jensen carried the torch to Copenhagen where the next I.C.C.C. was held three years later.
Spectacular New Development
From about 1950 interest in transition metal chemistry grew apace. When the fifth I.C.C.C. was held under the auspices of the Chemical Society in London in 1959 there were about 750 registrants and it has recently been agreed that the fiftieth anniversary conference will be held in London under the auspices of the Royal Society of Chemistry in the year 2000. What caused this sudden upsurge of interest during the 1950s? It was the development of the petrochemicals industry, especially after the war, with its supply of olefins together with the appearance of Reppe’s reports of some remarkable catalytic cyclisations and carbonylations of acetylenes and olefines carried out on certain nickel complexes (14). The following reaction particularly attracted attention:
This suddenly made influential organic chemists in industry aware of the potential of transition metal chemistry and loosened the purse strings of both Industry and Government. Thus although my work in the I.C.I. Laboratory and that of R. S. Nyholm at University College London, and of others overseas stood on the base of what had been initiated in academic laboratories before the war, it was now accepted more favourably by those controlling finance; at last transition metal chemistry started to flourish. We must not forget that Ronald Nyholm was a great propagandist who preached loud and long to the great benefit of our science as did John C. Bailar, Jr. and W. C. Fernelius in the U.S.A. Then came the discovery of the cyclopentadienyls (15), not particularly useful in themselves, but they brought transition metals even more into the realm of organic chemistry and so to the attention of a greater section of the chemical community. The greatest single event which finally revitalised transition metal chemistry was Ziegler’s discovery of the low pressure polymerisation of ethylene to a high polymer on various early transition metal compounds activated by aluminium alkyls (16) followed quickly by the discovery of isotactic polymerisation by Natta (17) in the 1950s. The remarkable production of stereo-regular polymers of propene and such olefins on titanium-aluminium alkyl catalysts fired the imagination of industry. So there was a great blossoming of research into the chemistry of transition metals in the late 1950s and early 1960s, especially into the area of the platinum metal complexes and their use in homogeneous catalysis (18). Despite increased financial stringency, it is still flourishing today as testified by the recently established series of international conferences devoted entirely to platinum metal chemistry.
What of the Future?
Catalysts continue to be developed, but there are more wonderful substances to be discovered in the realm of platinum metal chemistry: new and more highly specific catalysts, complexes to effect useful photosplitting of water perhaps, or a cure for cancer. We cannot tell. The most spectacularly important discoveries are often made by accident and so we must continue to probe the unknown. Speculative research is slow with its rewards, and becomes slower as less remains to be discovered, but if your instinct is good I believe that you will eventually break into a new area.
When I started research all chemists who thought about it at all knew that the transition metals had vitually no hydride or organic chemistry because the bonds between transition metal atoms and hydrogen or alkyl or aryl groups were considered to be too weak to survive at ordinary room temperatures. Organoplatinum metal compounds had been sought by the well established methods of Main Group metal chemistry and found scarcely to exist. The relative position of organoplatinum metal chemistry in the 1930s and now is well illustrated by consideration of the space allotted to them relative to organomercury compounds in the two comprehensive textbooks of those periods. In the Goddards’ four volumes published 1928 to 1937 there are 190 pages devoted to mercury, but only one and a half to the platinum metals (19). It appeared self evident that there was nothing worth looking for in organoplatinum metal chemistry, hence the remarks of my Nobel Prize winning mentor, but look where it is now. In Wilkinson’s Comprehensive Organometallic Chemistry there are only 115 pages devoted to mercury, but 1,432 to the platinum metals (20). Scientific discovery is not yet predictable. If you have an exciting road to follow do not be put off by those who say that there is nothing at the end of it; they do not know. Persevere, and enjoy the excitement of exploring the unknown.
- 1 I. T. Millar, Biogr. Mem. Fellows R. Soc., 1984, 30, 409
- 2 P. Pfeiffer, “Anorganischen Chemie”, 1923, pp. 56, 170
- 3 A. Burawoy,, C. S. Gibson,, G. C. Hampson and H. M. Powell, J. Chem. Soc., 1937, (II), 1690
- 4 A. Werner, Z. anorg. Chem., 1893, 3, 267 ; see also Chem. News, 1907, 281 and Platinum Metals Rev., 1966, 10, ( 4 ), 140
- 5 K. A. Jensen, Z. anorg. allg. Chem., 1936, 229, ( 3 ), 225
- 6 F. G. Angell,, H. D. K. Drew and W. Wardlaw, J. Chem. Soc., 1930, (I), 349
- 7 C. W. Blomstrand, Die Chemie der Jetztzeit, Heidelberg, 1869; J. prakt. Chem., 1888, 38, ( 8 & 9 ), 352
- 8 S. M. Jörgensen, J. prakt. Chem., , 1890, 41, ( 9 & 10 ), 429
- 9 M. S. Kharasch and T. A. Ashford, J. Am. Chem. Soc., 1936, 58, ( 9 ), 1733
- 10 A. E. Philbrick, and E. J. Holmyard,, “A Textbook of Theoretical Chemistry”, J. M. Dent and Sons, London, 1941, p. 528
- 11 J. Chem. Soc., Abstracts, 1893, LXIV, Part 2, 379
- 12 J. Chatt and L. A. Duncanson, J. Chem. Soc., 1953, (III), 2939
- 13 Current Contents, 1983, 23, ( 21 ), 20
- 14 W. Reppe and W. J. Schweckendiek, Justus Liebigs Ann. Chem., 1948, 560, 104 ; W. Reppe, “Acetylene Chemistry”, Charles A. Meyer & Co., New York, 1949
- 15 S. A. Miller,, J. A. Tebboth and J. F. Tremaine, J. Chem. Soc., 1952, (I), 632 ; T. J. Kealy and P. L. Pauson, Nature, 1951, 168, ( 4285 ), 1039 ; G. Wilkinson,, M. Rosenblum,, M. C. Whiting and R. B. Woodward, J. Am. Chem. Soc., 1952, 74, ( 8 ), 2125 ; E. O. Fischer and W. Pfab, Z. Naturforsch., 1952, 7b, ( 7 ), 377
- 16 K. Ziegler,, E. Holzkamp,, H. Breil and H. Martin, Angew. Chem., 1955, 67, ( 19 / 20 ), 541
- 17 G. Natta, J. Polym. Sci., 1955, 16, ( 82 ), 143 ; G. Natta,, P. Pino,, P. Corradini,, F. Danusso,, E. Mantica,, G. Mazzanti and G. Moraglio, J. Am. Chem. Soc., 1955, 77, ( 6 ), 1708
- 18 For a recent brief text see C. Masters, “Homogeneous Transition-metal Catalysis”, Chapman and Hall, London, 1981
- 19 A. E. & D. Goddard in “A Text Book of Inorganic Chemistry, Vol. 11. Organometallic Compounds”, ed. Newton Friend, Griffin & Company, London, 1928 –1937, Parts I & IV
- 20 “Comprehensive Organometallic Chemistry”, ed. G. Wilkinson,, F. G. A. Stone and E. W. Abel, Pergamon Press, Oxford, 1982, Vols. 4, 5, 6 & 8