Density functional theory (DFT) is applied to the calculation of activation of the Co-C-R bond in models of vitamin B-12, B-[Co-III(corrin)]-R. It is shown that there is a positive correlation of the bond lengths between the Co atom and the two axial ligands, B and R. The electron donation from axial ligands to the cobalt atom either by electron donating substituents or by a properly oriented external electric field caused by external electric charges is argued to be the main trigger for the activation of the Co-C-R bond.