Is a Transition State Planar or Nonplanar in Oxidative Additions of C−H, Si−H, C−C, and Si−C σ-Bonds to Pt(PH₃)₂? A Theoretical Study

A theoretical study of oxidative additions of H−CH 3, CH 3 −CH 3, H−SiR 3, and SiR 3 −CH 3 (R H, Cl, or Me) to Pt(PH 3 ) 2 was carried out with ab initio MO/MP2-MP4SDQ, CCD, and CCSD methods. The oxidative addition reactions of C−H and Si−H σ-bonds occur through a planar transition state (TS) structure, in accordance with the expectation from an orbital interaction diagram. However, the oxidative addition reactions of CH 3 −CH 3 and SiH 3 −CH 3 take place through a nonplanar TS structure, unexpectedly; the dihedral angle δ between PtP 2 and PtXC planes (X = C or Si) is about 70° for X = Si and about 80° for X = C. Intrinsic reaction coordinate calculation of the SiH 3 −CH 3 oxidative addition clearly indicated that this nonplanar TS is smoothly connected to the planar product on the singlet surface. The dihedral angle δ at the TS is larger in the SiMe 3 −CH 3 and SiCl 3 −CH 3 oxidative additions than that in the SiH 3 −CH 3 oxidative addition. Electron distribution in the TS and effects of bulky substituent on the dihedral angle suggest that not an electronic factor but a steric factor is responsible for the nonplanar TS structure of the C−C and Si−C oxidative addition reactions.