The B-12 cofactors methylcobalamin (MeCbl) and 5'-deoxyadenosylcobalamin (AdoCbl) have long fascinated chemists because of their complex structures and unusual reactivities in biological systems; however. their electronic absorption Abs) spectra have remained largely unassigned. In this study, we have used Abs, circular dichroism (CD), magnetic CD (MCD), and resonance Raman spectroscopic techniques to probe the electronic excited states of Co(3+)Cbl species that differ with respect to their upper axial ligand, including MeCbl, AdoCbl, aquacobalamin (H(2)OCbl(+)), and vitamin B-12 (cyanocobalamin, CNCbl). Also included to probe the effect of the lower axial ligand on the electronic properties of Cbls is Adocobinamide (AdoCbi(+)), an AdoCbl derivative that lacks the tethered base 5,6-dimethylbenzimidazole (DMB) and instead binds a water molecule in the lower axial position. Spectroscopic data for each species are analyzed within the framework of time-dependent density functional theory (TD-DFT) to assign the major spectral features (the so-called alpha/beta, D/E, and gamma bands) and to generate experimentally validated electronic-structure descriptions. These studies reveal that the "unique" Abs spectra of MeCbl and AdoCbl, which differ considerably from the "typical" Abs spectra of H(2)OCbl(+) and CNCbl, reflect the high degree of sigma-donation from the alkyl ligand to the Co center and the consequent destabilization of all Co 3d orbitals. They reveal further that with increasing sigma-donor strength of the upper axial ligand, the contribution from the formally unoccupied Co 3d,2 orbital to the HOMO increases, which induces a strong Co-N-DMB sigma-antibonding interaction, consistent with the experimentally observed lengthening of this bond from H(2)OCbl(+) to CNCbl and MeCbl. Alternatively, our spectroscopic and computational data for MeCbl and MeCbi(+) reveal that substitution of the DMB by a water molecule in the lower axial position has negligible effects on the Co-C bond. A simple model is presented that explains why the identity of the upper axial ligand has a major effect on the Co-N-ax bond strength, whereas the lower axial ligand does not appreciably modulate the nature of the CO-C bond. Implications of these results with respect to enzymatic Co-C bond activation are discussed.