Dihydrogen addition to (P(i)Pr-3)(2)OsXnH4-n

Reaction of H-2 with L2Os(X)(Y)(H)(2) (L=(PPr3)-Pr-i; X, Y = Cl, Br, I) slowly (minutes) gives a 1:1 'adduct'. The X-ray structure of L2OsH4Br2 shows, like the chloride analog, a substantial change in geometry upon reaction with H-2, to give a heavy atom OsX2L2 skeleton of C-2v geometry, with cis halides and trans phosphines. Attempts are made to distinguish between Os(H)(4), Os(H-2)(2) and Os(H)(2)(H-2) structures using J(HD) and T-1min criteria. Study of L2OsXnH4-n species shows no monotonic trend in the Delta G(double dagger) for H-2 loss, being higher for both n=0 and n=2 than for n=1. Studied as a function of X(=Cl, Br, I), H-2 binding to L2Os(H)(2)X-2 is favored enthalpically but disfavored entropically for the larger, least electronegative halide. The X-ray structure of L2OsDCl3 shows this unsaturated species to have significant angular distortions from octahedral geometry, consistent with its diamagnetic character. (C) 1997 Elsevier Science S.A.