A THEORETICAL-STUDY ON THE OXIDATIVE ADDITION OF A SI-H SIGMA-BOND TO[MCL(CO)(PH3)(2)] (M=RH OR IR) - SIMILARITIES TO AND DIFFERENCES FROM[M'(PH3)(2)] (M'=PD OR PT) AND [RHCL(PH3)(2)]

The Si-H Oxidative Addition to [MCl(CO) (PH3)(2)] (M = Rh or Ir) was t heoretically investigated with ab initio MO/MP2-MP4, SD-CI, and CCD (c oupled cluster with double substitutions) methods. This reaction proce eds with a somewhat high activation energy (E(a)) and a moderate exoth ermicity (E(exo)); when the oxidative addition occurs in the Cl-M-CO p lane, E(a) = 19 kcal mol(-1) for Rh and 13 kcal mol(-1) for Ir, and E( exo) = 0.4 kcal mol(-1) for Rh and 17.4 kcal mol(-1) for M = Ir. This result is in significant contrast to [M'(PH3)(2)] (M' = Pd Or Pt) and [RhCl (PH3)(2)] :the Si-H oxidative addition occurs with a very small barrier for [M'(PH3)(2)], but a zero barrier and significantly high ex othermicity for [RhCl(PH3)(2)]. The low reactivity of [MCl(CO)(PH3)(2) ] is interpreted in terms of the low energy level of the d orbital, a considerable weakening of the M-Cl and M-CO bonds, and an exchange rep ulsion between SiH4 and the M d orbital in [MCl(CO)(PH3)(2)]. The abov e results indicate that Vaska-type complexes are less favorable for an oxidative addition reaction than [M'(PH3)(2)] and [RhCl(PH3)(2)], and that they cannot be used for a catalytic reaction including an oxidat ive addition which requires a high activation energy.