New tetradentate ligands with N 2 OS coordination spheres were prepared and the corresponding nickel(II) complexes were synthesized and characterized by analytical, spectroscopic and electrochemical methods. The structure of nickel{methyl-2-{N-[2-(3 -methoxy-2 -phenolate)methylidynenitrilo]ethyl}am inato(-1)-1-cyclopentenedithiocarboxylate} has been determined by X-ray crystallography. This complex crystallizes in the orthorhombic space group P2 t 2 t 2 with a = 11.417 (3), b = 13.883 (4), c = 12.769 (3), Z = 4 and R 1 = 0.0539 for 4641 reflections with I > 2σ(I). The UV-Vis and 1 H NMR spectroscopic results indicate that the complexes have a square-planar structure in the solid state and in solution. Electrochemical and EPR data show that the complexes are reduced to the correspondent nickel(I) complexes, but that the ligands are non-innocent in the electrochemical oxidation of the complexes.