Density functional theory (DFT) has been applied to investigate the relationship between steric and electronic properties of the trans axial base and energetics of Co-C bond cleavage in models of coenzyme B-12. By using structurally reliable six-coordinate models, B-[Co-III(corrin)]-R+, it was shown that for a given base (B) the energy of homolytic cobalt-carbon bond cleavage correctly follows the CO-CR bond lengthening. For a given axial ligand (R) the dissociation energy is very weakly dependent on the trans axial base and correlates with its basicity. Analysis of the five-coordinate homolysis products, B-[Co-II(corrin)](+), shows that the Co-II- N-B bond length is in the range of similar to2.2 Angstrom, slightly shorter in comparison to six-coordinate analogues, while five-coordinate heterolysis products, B-[Co-III(corrin)](+2), have the Co-III-N-B bond significantly shorter similar to1.9 Angstrom. This noticeable difference suggests that controlling the Co-N-B bond length could be an effective way to promote biologically important homolysis of the Co-C bond in B-12-dependent enzymes over abiological heterolysis.