Density functional study on the mechanism of the C-X (X = Sn, Ge, Si, C, H) or bond oxidative addition of HCCR (R = SnH3, GeH3, SiH3, CH3, H) to the (PH3)2M (M = Ni, Pd, Pt) complexes.: Does the substrate approach the metal in a parallel or perpendicular manner?

The activation mechanism of the C-X (X = Sn, Ge, Si, C, H) sigma bonds of HCdropCR (R = SnH3, GeH3, SiH3, CH3, H) is theoretically examined with a density functional method (B3LYP) using the model complexes (PH3)(2)M (M = Ni, Pd, Pt) from the viewpoint of the parallel and perpendicular approach of the incoming substrate. For X = Ge, Si, C, H, the C-X a bonds approach the Pd parallel to the P-Pd-P plane and are activated in the P-Pd-P plane, as is well-known already for the H-H and C-H bond activation of the H-2 and CH4 molecules. In contrast, in the case of the highly polarized C-Sn sigma bond, the C-Sn sigma bond approaches the Pd perpendicularly to the P-Pd-P plane and the activation reaction proceeds by two processes via the resting state 2, having eta(2)-C-Sn bonding with the Pd. This is obviously different from the conventional or bond activation in the P-Pd-P plane. The entire potential energy surface for X = Sn was quite smooth, requiring a small energy barrier of only 0.8 kcal/mol due to the attractive interaction of Sn with the Pd at the apical site during the reaction. The difference among the three metals in the same group, Ni, Pd, and Pt, on the C-Sn sigma bond activation is also discussed.