Density Functional Theory (DFT) and Quantum Theory of Atoms in Molecules (QTAIM) calculations on [NH3[Co(III)(C10-X-corrin)]-CH3](+), where the C10-H of corrin is replaced by electron-donating or -withdrawing groups, X, explore the cis influence in these vitamin B-12 models. As the electron donating ability of X increases, a normal trans influence is observed: the Co-NH3 bond weakens while the Co-CH3 bond strengthens. Surprisingly, though, the Co-CH3 bond dissociation energy decreases monotonically as the Co-C bond strengthens. This is found to be a consequence of the extent of shortening of the Co-NH3 during the homolysis reaction. (c) 2012 Elsevier B.V. All rights reserved.