The title Schiff base ligand, H2L, and its complex [CuL] have been prepared and characterized by elemental analysis and i.r. spectroscopy. The electrochemical properties of the single crystal [CuL]·0.5C2H4Cl2 have been investigated by cyclic voltammetry (c.v.). Upon scanning at negative and positive potentials, two reduction and two oxidation waves were observed at E 1/2 0 = −0.47 and −0.77 V and E 1/2 0 = 1.10 and 1.39 V, respectively. Two K com values for the reduction and oxidation process are 1.2×105 and 8.0×104, respectively, and indicate that the deprotonated ligand, L, has the ability to stabilize the mixed valent states of both CuIICuI and CuIICuIII. The crystal structure has been solved by X-ray diffraction, with a final R = 0.0653. Wound around a copper atom, the effectively tetradentate chain ligand, L, coordinates only via its two imino and two phenolate groups at both ends of the chain. The geometry, D 2d symmetry, is best described as trans 'N2O2' square-planar with slight tetrahedral distortion. The reason, why the mononuclear [CuL] unit can display two c.v. waves in each process, has been explained on the basis of configuration interchange between mono- and dinuclear units in c.v. electrolyte solution.