Cis Labilization of Ligand Dissociation. 5. A Molecular Orbital Investigation

The process of ligand dissociation from a transition metal carbonyl center has been examined using a nonempirical molecular orbital approach. The energy requirements for CO dissociation from Mn(CO)6+, and from Mn(CO)5X, where X=Br or H, have been investigated. Consideration of the interaction between individual CO groups and the remainder of the molecule in the ground state structures of these species does not provide a rationale for the relative rates of CO dissociations. Instead, the relative energies for CO dissociation appear to depend upon the relative energies of stabilization resulting from geometrical relaxation of the metal carbonyl fragment which remains following CO loss. The cis CO groups of Mn(CO)5Br are found to be more labile than the trans CO group or than the CO groups of Mn(CO)6+, because in formation of the transition state more stabilization energy derives from relaxation of the Mn(CO)4Br fragment. Ligands which have at least one potential π-donor orbital labilize the cis carbnyl groups to the greatest extent, as a result of an improved bonding interaction in the five-coordinate fragment. The calculations suggest that the σ-bonding ability of a ligand such as H is less influential than π-bonding characteristics.