THEORETICAL-STUDIES OF INORGANIC AND ORGANOMETALLIC REACTION-MECHANISMS .7. ABINITIO POTENTIAL-ENERGY SURFACES OF CO SUBSTITUTIONS ON 6-COORDINATE TRANS-W(CO)4(NO)CL AND RE(CO)5CL

Carbonyl substitution reactions on trans-W(CO)4(NO)Cl and isoelectronic Re(CO)5Cl wcre investigated through the construction of ab initio potential energy surfaces. Hartree-Fock-Roothaan calculations for the substitution by PMe3 on W(CO)4(NO)Cl predict an associative mechanism with a 7-coordinate intermediate, in which the W-N-O angle is bent to 135.7-degrees. 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. For a weaker donor, such as PH3, this associative mechanism is not available because poor donors cannot stabilize an intermediate with a bent W-N-O bond. Substitution by any PR3 in Re(CO)5Cl also proceeds by a dissociative or I(d) Mechanism, since here the CO ligand cannot accommodate an additional electron pair and it resists bending to maintain a strong Re-C bond. We also explored the influences of electron correlation and basis set on the potential energy surfaces.