SIH, SISI, AND CH BOND ACTIVATION BY COORDINATIVELY UNSATURATED RHCL(PH3)2 - AB-INITIO MOLECULAR-ORBITAL STUDY

SiH bond activation, oxidative addition of the SiH bond of SiH4 to coordinatively unsaturated RhCl (PH3)2, was theoretically investigated with an ab initio molecular orbital (MO) method, and its potential energy profile was compared with that for CH bond activation, oxidative addition of the CH bond of CH4. All the stationary points were determined at the MP2 level. While CH bond activation passes through an eta2-CH4 complex and a three-centered transition state, SiH bond activation is downhill and the eta2-SiH4 complex is a transition state for intramolecular rearrangement connecting two silyl hydride complexes, the products of SiH bond activation. This difference originates from the much larger exothermicity of SiH bond activation due to the weaker SiH bond and the stronger RhSi bond compared with the C counterparts. The RhSi bond is about 20 kcal/mol stronger than the RhC bond; the large electron donation from SiH3 to Rh can take place in the late transition metal complex to stabilize the electropositive silyl group. However, the order in M-R bond strength depends on the transition metal. In the Zr complex, a strong back-donation from Zr to CH3 takes place because of the small electronegativity of Zr, resulting in the ZrC bond being stronger than the ZrSi bond. For Si2H6, activation of both the SiH and the SiSi bond takes place easily, and RhCl(PH3)2(SiH3)2 and HRhCl(PH3)2(Si2H5) can rearrange with each other intramolecularly. Also, the electron correlation effect on structure and energetics is discussed.