Density functional theory (DFT) Becke3LYP calculations including full and restricted geometry optimizations are carried out on the complex [Co(Cor)(Benz)(CH3)](+) (Cor = corrin, Benz = benzimidazole), which is a model of B-12 cofactors, and on the products of the two possible heterolytic cleavages of the Co-C bond, [Co(Cor)(Benz)(CH3)], CH3+, [Co(Cor)(Benz)(CH3)](2) (+), and CH3-. The thermodynamics of the heterolytic processes are found to depend very significantly on the distance of the axial ligand from the cobalt. The results are explained through simple molecular orbital reasonings, and their possible implications for the biological reactivity of adenosylcobalamin and methylcobalamin are discussed.