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REACTIVITY PATTERNS OF TRANSITION METAL POLYHYDRIDES

The majority of both fundamental and applied studies of transition metal hydride chemistry are based upon compounds with a metal oxidation state of two and below. Work with the Rhl/Rh"' and Ir'/Ir'" couples exceeds this threshold, but these trivalent complexes generally have no more than two hydride ligands. For the purposes of this discussion, we will arbitrarily refer to L,MH, complexes with n 2 3 as high-ualent polyhydrides. As shown in TABLE 1, compounds of this stoichiometry are abundant. Moreover, they dramatically illustrate the broad range of oxidation states tolerated by phosphine and also hydride ligands. The compounds shown have been isolated for a range of tertiary phosphine ligands,' but little is known concerning their reactivity. All compounds shown possess an 18 valence electron configuration. Of particular interest are the low d-electron populations, including d 2 and even do. We began this study anticipating that high-valent hydride complexes might exhibit new organometallic reaction patterns (and perhaps mechanisms) involving the catalytically relevant hydride ligand. For example, is it possible that activation of aliphatic hydrocarbons can be achieved by electrophilic attack? This is suggested by the successful H/D exchange using Pt(Il)/Pt(IV) in acidic medium., Since we have the potential for generating formally high-valent unsaturated polyhydrides, will these effect C-H and C-C bond scission? Our work has thus been directed toward exploring and hopefully exploiting the potentially unique reactivity of high-valent metal polyhydrides.