New dinuclear nickel(II) complexes of formula [(NiL)(2)ox](ClO4)(2) (ox(2-)-oxalate anion, L = 3-methyl-1,3,5,8,12-pentaazacyclotetradecane, complex 1; L = 3,10-(2-hydroxyethyl)-1,3,5,8,10,12-hexaazacyclotetradecane, complex 2; and L = 1,4,8,11-tetraazaundecane, complex 3 have been synthesized and characterized by means of absorption and IR spectroscopy. Electrochemical studies of 1-3 in DMF and DMSO indicated high stability of the mixed-valence species (Ni-II-Ni-III). The values of K-com and Delta G(com) of the comproportionation equilibrium (Ni-II-Ni-III) + (Ni-III-Ni-III) = 2(Ni-II-Ni-III), were determined by cyclic voltammetry, and compared with those for the complex [(NiL4)(2)ox](2+) (L-4 = 1,4,8,11-tetraazacyclotetradecane, cyclam). The main factor contributing to the stabilization of the mixed-valence state was ascribed to the electronic delocalization through the bridging oxalate ligand. The influence of the structure of amine ligands on the values of K-com was discussed. The electrochemical behaviour of dinuclear complexes was compared with that of the parent mono-valent species. Polynuclear transition metal complexes represent an interesting class of coordination compounds which often display intriguing spectroscopic, magnetic and electrochemical properties. Such compounds attract much attention because of their functions associated with metal-metal interactions [1] and as models for metal biosites [2]. The complexes of linear and macrocyclic polyamines serve as convenient building blocks for the construction of such polynuclear tin the simplest case dinuclear) aggregates. These compounds possess very high thermodynamic stability, kinetic inertness and the ability to change easily the oxidation state of the coordinated metal ions.