Molecular orbital analysis of anomalous trans effect in cobalamins

Density functional theory has been applied to the analysis of NB-Co-CR interligand bonding in octahedral complexes of CoIII. Employing B-[CoIII(corrin)]-R+ models, it is shown that a change in the electron donating (or withdrawing) character of alkyl ligands (R) lengthens (or shortens) both interligand Co-CR and Co-NB bonds in accordance with the anomalous (or inverse) trans effect. Six molecular orbitals are required for the succinct description of this phenomenon, which lies in an unusual combination of a poor σ/π-donor of the base (B) and an unusually strong σ-donor of the alkyl group (R).