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 t o 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-t o 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 int eraction's strength depends upon the M ... H overlap, and stronger ago stic 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 a gostic complex. The ethylene rotation barrier (calculated to be 9.8 kc al mol-1) depends greatly on the sizes of the transition metal atom an d ligands in the complex since steric effects play an important role i n the rotation.