MIXED VALENCY IN BINUCLEAR CYANO-BRIDGED MANGANESE BIS(CARBONYL) COMPLEXES AND STEREOCHEMICAL CONTROL OF THEIR OXIDATION - A MOLECULAR-ORBITAL STUDY

Molecular orbital calculations at the extended Huckel level have been carried out on the model binuclear cyano-bridged cationic complexes of the type {[Mn]-CN-[Mn]}+ where [Mn] = cis- or trans-mer-Mn(CO)2(PH3)3+. The results are consistent with a weak Mn- - -Mn interaction mediated by the CN bridge. The energy and composition of the highest occupied orbitals are determined by the stereochemistry and so is the Mn atom at which the HOMO is localized. A good correlation is found between these data and the experimental electrode potentials and the effects of the ancillary ligands corresponding to the real complexes with [Mn] = cis- or trans-mer-Mn(CO)2(diphos)L (diphos = dppm or dppe, L = PR3 or P(OR)3). An explanation is also found for the fact that the oxidatively induced isomerization of the cis-Mn(CO)2 fragments occurs even when the oxidation is centered at the other Mn center. The calculations also indicate that the dications formed upon the first oxidation can be considered as very weakly coupled class II mixed-valence complexes, with a different degree of electron delocalization depending on the stereochemistry of the Mn(CO)2L3 fragments bridged by the cyanide ligand.