Acetylene-insertion reactions into Pt(II)-H and Pt(II)-SiH3 bonds. An ab initio MO study and analysis based on the vibronic coupling model

Acetylene insertion into Pt(II)-H and Pt(II)-SiH3 bonds of PtH(SiH3)(PH3) w as investigated using ab initio molecular orbital and Moller-Plesset pertur bation theory methods. The insertion into Pt-H was predicted to proceed wit h a smaller activation energy (E-a = 12.8 kcal/mol) than that into Pt-SiH3 (E-a = 20.9 kcal/mol). The reaction energy (Delta E) of the insertion into Pt-H is 10 kcal/mol smaller than that for the insertion into Pt-SiH3, which reflects differences in bond energies between C-H and C-Si and between Pt- H and Pt-SiH3. A comparison with ethylene insertion revealed that the acety lene insertion occurs more easily, and the latter reaction is more exotherm ic. A simple vibronic coupling model combined with Toyozawa's interaction m ode analysis was used to examine interesting differences in E-a and Delta E between insertions into Pt-H and Pt-SiH3, and between acetylene and ethyle ne insertions. This analysis suggests that the factors determining E-a are the stiffness of the Pt-H and Pt-SiH3 bonds and the vibronic coupling stren gth of acetylene and ethylene.