An ab initio molecular-orbital study of insertion of CO2 into a RhI–H bond
Abstract
The insertion of CO2into the RhI–H bond of [RhH(PH3)3] was investigated theoretically by the ab initiomolecular-orbital method, in which geometries of the reactants, the transition state and products were optimized at the Hartree–Fock level, and MP4SDQ, SDCI 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 CuI-H bond of [CuH(PH3)2](Ea= 3.5 kcal mol–1 and Eexo=ca. 40 kcal mol–1), calculated at the SDCl level. The lower exothermicity arises from the fact that the RhI–H bond is much stronger than the CuI-H bond. The higher activation energy is interpreted in terms of the stronger RhI-H bond, the weaker electrostatic stabilizing interaction and the stronger exchange repulsion interaction between CO2and [RhH(PH3)3]in the transition state. Owing to this strong exchange repulsion, the transition state does not contain a four-centra type interaction.