The structures of methyl-, (5’-deoxyadenosyl)-, benzyl-, and neopentylcobalamin were determined by molecular mechanics calculations using a force field developed specifically for the cobalt corrinoids. A grid search of the cobalamin ring side-chain orientations in MeCbl was used to identify their accessible conformations in the temperature range in which thermolysis of the alkylcobalamins have been studied. Rotation of the alkyl ligand in each complex was used to identify its possible orientations relative to the corrin ring. Thus, likely conformations of each complex were identified, and from these, the population-weighted Co-C bond lengths and Co-C-X (X = C, H) bond angles were determined. The steric strain induced in each complex on coordination of the alkyl ligand was investigated by determining the steric strain as a function of Co-C bond distance. The steric demands of the alkyl ligands were found to be Me < Bz approximate to Ado < Np, and the Co-C bond length and Co-C-X (X = H in MeCbl, X = C in AdoCbl, BzCbl, and NpCbl) angle increases with an increase in the steric demand. There is an inverse relationship between the bond dissociation energy for the Co-C bond and the steric strain in the complex and the Co-C bond length and the Co-C-C bond angle for MeCbl, AdoCbl, and NpCbl (but not BzCbl because of resonance stabilization of the incipient radical). This suggests that the structure of the ground state is influenced by the same factors that control the strength of the Co-C bond in alkylcobalamins. The implication of these findings for the AdoCbl-dependent enzymes is discussed.