In summary, high-quality DFT calculations and judicious use of group theory have led to significant insights into the question of metal-versus ligand-centered redox in Co and Ni tetradehydrocorrin complexes. For the +1 states, both metals occur in their low-spin M(II) forms. In contrast, the charge-neutral states vary for the two metals: while Co(I) and Co II -TDC state are comparable in energy for cobalt, a Ni II -TDC •2-state is clearly preferred for nickel. The latter behavior may be contrasted with other corrinoids that reportedly do stabilize a Ni(I) center.