The homobinuclear cation [Ni-2(L-1)](4+) (L-1 = 6,6'-C-spirobi(cyclam)), in which the nitrogen donors adopt a trans-III configuration, has been synthesized, and its redox behavior has been investigated by electrochemical, spectroscopic, and kinetic methods. Cyclic voltammetry in aqueous media ([H+] = 0.1 and 1.0 M in LiNO3/LiClO4/CF3SO3Li) shows a single wave (Delta E-p = 58 mV). With the gradual addition of LiCl/Li2SO4, this wave is shifted cathodically. Simultaneously, the value of Delta E-p also gradually decreases from 58 to 37 mV in 1.0 M chloride and to 42 mV in 0.75 M sulfate media. Similar trends are also noticed in linear df voltammograms. [Ni-2(L-1)](ClO4)(4) in CH3CN ([TEAP] = 1.0 M; E-1/2(1) = 0.735 V and E-1/2(2) = 0.835 V vs Fc/Fc(+)) and [Ni-2(L-1)]Cl-4 in CH3Cl2 ([TEAC] = 0.1 M; E-1/2(1) = 0.640 V and E-1/2(2) = 0.766 V vs Fc/Fc(+)) media show two overlapping waves, In nitromethane, the redox process is complicated by adsorption. Because of the very small difference in redox potentials, the selective oxidation of one of the Ni(II) centers to form a mixed-valent Ni(II)-Ni(III) species is not practical. However, during reductive decomposition of the his-Ni(LII) species, a mixed-valent species could be identified. In stopped flow kinetics experiments, the UV-visible spectrum of a transient Ni(II)-Ni(III) intermediate in acetonitrile has been elucidated by one-electron reduction of the bis-Ni(III) species with [Ni-II(9-aneN(3))(2)](2+) (9-aneN(3) = 1,4,7-triazacyclononane). The differences in the redox behavior in various media are explained in terms of electrostatic interactions between the adjacent nickel centers influenced by sterically controlled axial ligation of counterions and solvents. Evidence for changes in coordination geometry in Ni(II) and Ni(III) species obtained from UV-vis and ESR studies is also presented.