Stabilised Carbonium Ions in the Organic Chemistry of Platinum THE EVIDENCE FOR THEIR EXISTENCE

Historically, platinum has played a prominent role in organometallic chemistry. The compounds KPtCI,(C,H,)H,O (I), PtMe, (X) (2) and Chugaev's salt [PtC,H,,N,],Clx (3, 4) present some of the fist examples of stable metal-olefin, metal-alkyl and metalcarbene complexes, although ironically the nature of the bonding in these compounds was not immediately recognised. In the study of organometallic reaction mechanisms platinum has also been in the forefront, for example, in studies of the decomposition pathways of metal-alkyls (5, 6); oxidativeaddition reactions (7-9); C-C and C-H reductive-elimination reactions (10-14); and M-H and M-C insertion reactions (15-22). Certain aspects of the organometallic chemistry of platinum are well suited for the study of organic reactions occurring at metal centres (23) : (i) There exists a diverse organometallic chemistry for platinum in its well-defined valence states 0, 2+ and 4f. (ii) For a given valence state platinum shows a well-defined coordinated chemistry. Pt(o) compounds are either three or four coordinate. Pt(2+) compounds show a remarkable preference for a four coordinate square planar geometry; five coordination is comparatively rare. Compounds of Pt(4-k) are almost invariably six coordinate. (iii) Platinum-carbon bonds are stable to oxygen, water and thermolysis. This stability results from a combination of thermodynamic and kinetic factors. (iv) Platinum compounds are well suited to a wide variety of spectroscopic techniques. In particular the presence of 19TtJ I=:&, 34 per cent natural abundance, provides a useful mechanistic probe in n.m.r. studies of organoplatinum compounds. This account summarises work in our laboratories which indicates that platinum may induce carbonium ion reactivity in ~ i bonded hydrocarbon ligands and may stabilise carbonium ions in the reactions of o-bonded unsaturated hydrocarbons.