Nitrosylcobalamin (NOCbl) is readily formed when Co(II)-balamin reacts with nitric oxide (NO) gas. NOCbl has been implicated in the inhibition of various B-12-dependent enzymes, as well as in the modulation of blood pressure and of the immunological response. Previous studies revealed that among the known biologically relevant cobalamin species, NOCbl possesses the longest bond between the Co ion and the axially bound 5,6-dimethylbenzimidazole base, which was postulated to result from a strong trans influence exerted by the NO ligand. In this study, various spectroscopic (electronic absorption, circular dichroism, magnetic circular dichroism, and resonance Raman) and computational (density functional theory (DFT) and time-dependent DFT) techniques were used to generate experimentally validated electronic structure descriptions for the "base-on" and "base-off" forms of NOCbl. Further insights into the principal Co ligand bonding interactions were obtained by carrying out natural bond orbital analyses. Collectively, our results indicate that the formally unoccupied Co 3d(z)(2) orbital engages in a highly covalent bonding interaction with the filled NO pi* orbital and that the Co-NO bond is strengthened further by sizable pi-backbonding interactions that are not present in any other Co(Ill)Cbl characterized to date. Because of the substantial NO- to Co(III) charge donation, NOCbl is best described as a hybrid of Co(III)-NO- and Co(II)-NO center dot resonance structures. In contrast, our analogous computational characterization of a related species, superoxocobalamin, reveals that in this case a Co(III)-O-2(-) description is adequate due to the larger oxidizing power of O-2 versus NO. The implications of our results with respect to the unusual structural features and thermochromism of NOCbl and the proposed inhibition mechanisms of B-12-dependent enzymes by NOCbl are discussed.