AN AB-INITIO MOLECULAR-ORBITAL STUDY OF INSERTION OF CO2 INTO A RH-I-H BOND
The insertion of CO2 into the Rh-l-H bond of [RhH(PH3)(3)] was investigated theoretically by the ab initio molecular-orbital method, in which geometries of the reactants, the transition state and products were optimized at the Hartree-Fock level, and MP4SDQ, SDCl and coupled cluster calculations were carried out on those optimized structures. This reaction is calculated to occur with a higher activation energy (16 kcal mol(-1)) and lower exothermicity (24 kcal mol(-1)) than the similar insertion into the Cu-l-H bond of [CuH(PH3)(2)] (E(a) = 3.5 kcal mol(-1) and E(exo) = ca. 40 kcal mol(-1)), calculated at the SDCl level. The lower exothermicity arises from the fact that the Rh-l-H bond is much stronger than the Cu-l-H bond. The higher activation energy is interpreted in terms of the stronger Rh-l-H bond, the weaker electrostatic- stabilizing interaction and the stronger exchange repulsion interaction between CO2 and [RhH(PH3)(3)] in the transition state. Owing to this strong exchange repulsion, the transition state does not contain a four-centre type interaction.
Journal Of The Chemical Society-Dalton Transactions
Royal Society of Chemistry