Density functional theory has been applied to the analysis of N-B-Co-C-R interligand bonding in octahedral complexes of Co-III. Employing B-[Co-III(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-N-B 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 sigma/pi-donor of the base (B) and an unusually strong sigma-donor of the alkyl group (R). (c) 2005 Elsevier B.V. All rights reserved.