AB-INITIO MOLECULAR-ORBITAL STUDY OF THE MECHANISM OF H-H, C-H, N-H, O-H, AND SI-H BOND ACTIVATION ON TRANSIENT CYCLOPENTADIENYLCARBONYLRHODIUM

An ab initio molecular orbital method at the MP2 level of theory in co njunction with a relativistic core potential and valence triple-zeta polarization basis set for Rh and double-zeta + polarization basis se t for other atoms has been applied to the study of the potential energ y surface of the oxidative addition reaction CpRh(CO) + HR --> CpRh(CO )(H)(R), where HR is H-2, CH4, NH3, H2O, and SiH4. At gas-phase collis ionless conditions, the oxidative addition reaction of H-SiH3, H-H and H-CH3 to CpRh(CO) should take place without an activation barrier, wh ile the reaction of H-NH2 and H-OH goes over a barrier about 5 kcal/mo l relative to the reactants. The differences in the reactivity of the substrates considered here can be correlated to the H-R bond strength and the Rh-R bond strength as well as the exothermicity of reaction. G oing from SiH4 to H-2, CH4, NH3, and H2O, the H-R bond becomes stronge r (88, 99, 108, 109, and 118 kcal/mol, respectively, calculated at the present level), the Rh-R bond becomes weaker (73, 65, 59, 47, and 55 kcal/mol, respectively), the exothermicity becomes smaller (49, 31, 16 , 3, and 2 kcal/mol, respectively), and the ease of reaction decreases . In solution or in the gas phase when the collisional energy equilibr ium is faster than the reaction itself and reaction should be consider ed to start from the pre-reaction molecular complex CpRh(CO) (HR), the oxidative addition reaction of CH4 requires a small barrier (6 kcal/m ol), while that of NH3 and H2O requires a large barrier (42 and 26 kca l/mol, respectively) and would not take place easily under normal cond itions. The high barrier is essentially determined by the stability of the molecular complex.