ELECTRONIC-STRUCTURE OF MO(VI ALKYLIDENE COMPLEXES AND AN EXAMINATION OF REACTIVE INTERMEDIATES USING THE SCF-X-ALPHA-SW METHOD

SCF-Xalpha-SW calculations were carried out on Mo(VI) imido alkylidene complexes, Mo(NH)-(CH2)(OH)2 (1), Mo(NH)(CH2)(OCH3)2 (2), a complex in which the alkylidene ligand has been rotated 90-degrees (3), and the molybdacyclobutane complex, Mo(NH)(CH2CH2CH2)(OH)2(4). In the basic model (1) the HOMO is the relatively nonpolar Mo-C pi-bond; the contributions by the C 2p and the Mo 4d atomic orbitals are approximately equal. An agostic interaction involving the C-H(anti) bond in the methylene ligand in 1 can be identified. The alkoxide oxygen 2p orbitals contribute to a significant extent to most molecular orbitals in 1, and consequently the alkoxides strongly influence the reactivity of the complexes. The LUMO in 1 is the M-N pi-antibonding orbital in the alkylidene plane, which is largely metal-centered on the COO face of the pseudotetrahedral complex. The electronic structure of 2 is roughly the same as 1. Alkylidene rotation in 1 is explored and the evolution of molecular orbitals during this process is discussed. Bending of the imido proton toward the alkylidene ligand is an important stabilizing feature in the ''rotated'' alkylidene complex, lowering the activation energy for rotation by approximately 50%. Calculations suggest that 4 can form readily when ethylene approaches the COO face of 3.