Electronic structure and reaction mechanism of transition metal complexes : bond-stretch isomerism, methane activation on transient cyclopentadienylcarbonylrhodium, and ligand substitutions on metal nitrosyl carbonyl complexes

Abstract

Ab initio molecular orbital techniques are utilized to investigate the electronic structure and reaction mechanism of transition metal complexes. Our calculations at the levels of the restricted Hartree-Fock (RHF) and configuration interaction provide no evidence for the existence of bond-stretch isomers. For (LWOCl2) + (L=N,N'N''-trimethyl-1,4,7=triazacyclononane complexes, only the 2A' state can be identified as a ground state. The 2A'' is an excited state and the orbital crossing mechanism cannot explain the occurrence of two stable isomers. For cis-mer-MoOCl2(PR3)3 complexes, the second-order Jahn-Teller effect is too weak to cause the bond-stretch phenomenon. Methane oxidative-addition to a transient RhCp(CO) complex involves an agostic-like intermediate in the early stage of the reaction with a metal-CH dative interaction. The transition state shows C-H bond breaking and is stabilized by the Rh-C and Rh-H dative bonding interactions. The second-order Moller-Plesset perturbation calculations predict an exothermic reaction with a reaction energy of 30.6 kcal/mol, an intermediate with a stabilization energy of 14.8 kcal/mol and an activation barrier of 4.1 kcal/mol relative to the intermediate. CO substitution reactions on W(CO)4(NO)Cl and isoelectronic Re(CO)5Cl are examined through the construction of potential surfaces. The RHF calculations for the substitution by PMe3 on W(CO)4NOCl predict an associative mechanism with a 7-coordinate intermediate. The Laplacian of the total charge density displays a process in which electrons shift from the metal to the nitrogen, create an additional N lone pair and vacate a coordinate site for the entering ligand. Substitution by any PR3 on Re(CO)5Cl, however, proceeds by a dissociative or I[d] mechanism since the CO ligand cannot accommodate an additional electron pair. We also explored the influences of both the electron correlation and the basis set superposition error on the potential surfaces.