The electrochemical behavior of [Mn-III(terpy)(N-3)(3)] (1) (terpy = 2,2 ': 6 ' ,2 " -terpyridine), a structural model of the azide complex of the manganese superoxide dismutase (MnSOD), has been investigated in acetonitrile (CH3CN) solution. In CH3CN containing either 0.1 M tetra-n-butylammonium perchlorate (Bu4NClO4) or tetraethylammonium trifluoroacetate (Et4NCF3CO2) as supporting electrolytes, the cyclic voltammogram of 1 exhibits one quasi-reversible reduction wave at E-1/2 = - 0.170 V versus Ag / 10 mM Ag+ and one quasi-reversible oxidation wave at E-1/2 = + 0.675 V. These are both one-electron waves, corresponding to the Mn(III)/Mn(II) and Mn(III)/Mn(IV) redox couples respectively. To evaluate the stability of the oxidized and reduced species of 1, exhaustive electrolyses have been carried out. Controlled-potential reductions at -0.35V of solutions of 1 in CH,CN containing 0.1 M Bu4NClO4 or 0.1 M Et4NCF3CO2 lead to the quantitative conversion of 1 into the bridging N-3(-) dimanganese(II) Complex, [(N-3) (terpy)Mn-II(mu -N-3)(2)Mn-II(terpy) (N-3)] (2). This transformation is chemically reversible by an oxidation process. Controlled-potential oxidation at 0.8 V of a solution of 1 in CH3CN + 0.1 M Bu4NClO4 produces a new mononuclear Mn(IV) complex characterized by electron paramagnetic resonance spectroscopy, which is stable only at or below - 15 degreesC. If this oxidation is conducted in CH3CN + 0.1 M Et4NCF3CO2 the stable dimanganese(IV) di-mu -oxo complex [(CF3CO2)(terpy)Mn-IV(mu -O),Mn-IV(terpy)(CF3CO2)](2+) (3) is formed quantitatively owing to the presence of an excess of the coordinating CF3CO2- anions and residual H2O in the CH3CN solution.