Nickel superoxide dismutase (NiSOD) is a bacterial metalloenzyme that possesses a mononuclear Ni-center and catalyzes the disproportionation Of O2(center dot-) by cycling between Ni-II and Ni-III oxidation states. Herein we present evidence from several SOD active metallopeptide maquettes ([Ni((SODH)-H-M2(1)X)]; (SODH)-H-M2(1)X = H2N-XCDLPCG-COOH; X = H, D, or A) that the Ni-center of NiSOD most likely remains five-coordinate during SOD catalysis using thin-film voltammetry. N-3- and CN- titration studies suggest that O-2(center dot-) disproportionation by [Ni(SODM2 H(1)X)] proceeds via an outersphere mechanism. Computationally derived values for the nuclear reorganization energy of the [Ni-II(SODM2)]/[Ni-III(SODM2)] self-exchange reaction combined with the experimentally determined value for k(o) (similar to 450 s(-1)) suggest that axial ligation enhances the O-2(center dot-) disproportionation reaction in [Ni(SODM2)] (and NiSOD by analogy) by optimizing the Ni-II/Ni-III redox couple such that it is close to the midpoint of the O-2(center dot-) reduction and oxidation couples.