Density functional theory (DFT) Becke3LYP calculations including full and restricted geometry optimizations are carried out on the complexes [Co(Cor)(Benz)(CH3)] (Cor = corrin, Benz = benzimidazole), [Co(Cor)(Benz)], [Co(Cor)(CH3)], and [Co(Cor)]. These systems, despite the absence of side-chains, constitute the most realistic models used to date for DFT calculations on cofactor B-12 and its homolysis product. The calculations prove that both thermodynamics and kinetics of the homolytic bond cleavage of the Co-C bond have very little dependence on the position of the axial benzimidazole ligand. The generality of these results is confirmed by additional calculations on [Co(Cor)(Benz)(CH2R)] (R = tetrahydrofuran), [Co(Cor)(Im)-(CH3)] (Im = imidazole), and [Co(Cor-CH3)(Benz)(CH3)] (Cor-CH3 = methylated con-in).