THEORETICAL-STUDIES OF INORGANIC AND ORGANOMETALLIC REACTION-MECHANISMS .8. HYDROGEN-EXCHANGE IN THE BETA-AGOSTIC ETHYLENE COMPLEX OF CYCLOPENTADIENYL RHODIUM

Ab initio calculations with effective core potentials have been used to study the hydrogen exchange processes of the [CpRh(C2H4)(eta2-C2H5)]+ beta-agostic complex. The mirror-symmetric olefin-hydride species is found to be an intermediate (3.4 kcal mol-1 higher in energy than the beta-agostic complex) rather than a transition state (almost-equal-to 5 kcal mol-1) in the interconversion process of the two enantiomeric forms. A higher energy process involving rotation of the methyl group in the agostic complex is determined to occur through an Rh ... (eta2-H2C) interaction with a calculated activation energy of almost-equal-to 6.5 kcal mol-1. Complete loss of the Rh ... H-C agostic interaction requires an activation energy of about 14 kcal mol-1. This agostic interaction's strength depends upon the M ... H overlap, and stronger agostic interactions result in weaker C-H(agostic) bonds. An even higher energy process involving the exchange of all of the nine hydrogens and the four carbons is found to be due to the ethylene rotation in the agostic complex. The ethylene rotation barrier (calculated to be 9.8 kcal mol-1) depends greatly on the sizes of the transition metal atom and ligands in the complex since steric effects play an important role in the rotation.