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).