Theoretical study on sigma-bond activation of (HO)(2)B-XH3 by M(PH3)(2) (X= C, Si, Ge, or Sn; M = Pd or Pt). Noteworthy contribution of the boryl p(pi) orbital to M-boryl bonding and activation of the B-X sigma-bond

Oxidative addition of (HO)(2)B-XH3 to M(PH3)(2) (X = C, Si, Ge, or Sn; M = Pd or Pt) was theoretically investigated with MPS-MP4(SDQ) and CCSD(T) meth ods. (HO)(2)B-XH3 easily undergoes oxidative addition to Pt(PH3)(2) with a moderate activation energy for X = C and either a very small barrier or no barrier for X = Ge, Si, and Sn. Also, (HO)(2)B-SiH3, (HO)(2)BGeH3, and (HO) (2)B-SnH3 undergo oxidative addition to Pd(PH3)(2) with either a very small barrier or no barrier. Only the oxidative addition of (HO)(2)B-CH3 to Pd(P H3)(2) cannot take place, but the reductive elimination of(HO)(2)B -CH3 fro m Pd(CH3)[B(OH)(2)](PH3)(2) occurs with no barrier. The transition states ( TS) of these oxidative additions are nonplanar except for the nearly planar TS of the oxidative addition of(HO)(2)B-CH3 to Pt(PH3)(2). This TS structu re is very sensitive to steric and electronic factors; for instance, the TS becomes nonplanar by substituting PH2(C2H5) for PH3, to decrease the steri c repulsion between (HO)(2)B-CH3 and PH2(C2H5). A noteworthy feature of the se reactions is that the TS is much stabilized by the charge-transfer inter action between M d and B(OH)(2) p(pi) orbitals, which is the main reason fo r the high reactivity of (HO)(2)B-XH3 in the oxidative addition reaction. P t-B(OH)(2) and Pd-B(OH)(2) bonds are much stronger than Pt-XH3 and Pd-XH3 b onds, respectively. This is because the M-B(OH)(2) bond is stabilized by th e pi-back-donating interaction between the empty p(pi) orbital of B(OH)(2) and the doubly occupied d, orbital of Pt and Pd. Also, it should be noted t hat the trans influence of the boryl group is stronger than the very strong trans influence of silyl group.