Compared with metalloporphyrins and metallocorroles, cobalamin and F430 models remain less explored. An important recent development has been the availability of the monoanionic B,C-tetradehydrocorrin ligand and its coordination to cobalt and nickel states (Figure ). Both metals have yielded complexes formally at the M(I) state. In the nickel case, the neutral complex has been fairly conclusively assigned to a Ni II -TDC •2-state, but the nature of the neutral Co complex is more subtle. A neutral cobalt corrin has been traditionally thought of as a d 8 Co(I) complex; however, advanced ab initio calculations have suggested a more multiconfigurational description. Herein we have examined the low-energy states of C2-symmetrized M[TDC] (M = Co, Ni) complexes at the 0 and +/-charge with high-quality density functional theory (DFT) calculations. Symmetrization allowed us to calculate different electron occupancies for the two irreducible representations in question and thereby to determine the relative energetics of metal-versus ligand-centered redox in both the neutral and ionized states of the molecules. Such exercises have a long and successful track record vis-a-vis porphyrin-type molecules and have shed a good deal of light on electronicstructural aspects of manganese porphyrins, low-spin ferrihemes, cobalt dipyrrinbisphenolates, nickel porphyrins and hydroporphyrins, and metallocorroles including metal-metal-bonded metallocorrole dimers, as well as on the photoelectron spectra of porphyrins. The present exercise allows for greater certainty in the interpretation of redox behavior Co and Ni corrinoids as well as new insights into metal-ligand interactions in these systems. Our M[TDC] model is a slightly simplified and symmetrized analogue of the system experimentally studied by Lindsey, Nocera and their associates (Figure ). The simplification consists of merely replacing a meso-p-tolyl group with a phenyl group so as to generate a C2symmetric model. Two well-tested exchange-correlation functionals were generally used -OLYP and B3LYP*, the latter containing 15% Hartree-Fock exchange relative to B3LYP, which contains 20%. Both functionals were augmented with Grimme's D3 dispersion corrections. A spin-unrestricted formalism and all-electron ZORA STO-TZ2P basis sets were used throughout. Appropriately fine meshes for numerical integration of matrix elements were employed, as were suitably tight criteria for geometry optimizations. as previously observed, the pure functional OLYP exhibits a certain preference for spin-paired states, whereas the hybrid functional B3LYP*-D3 favors a greater degree of spin decoupling, i.e., classic behavior for the two classes of functionals. These results point to a multiconfigurational ground state for Co[TDC] and indeed for cobalamin and "Co(I)" corrinoids in general, with several low-energy excited states. As for why the TDC ligand fails to decisively stabilize a true Co(I) relative to a