A set of nine new tetradentate ligands were prepared and the corresponding nickel(II) complexes with co-ordination spheres NN′OS synthesized and studied by spectroscopic and electrochemical techniques. The structure of (methyl 2-{[3-(2 hydroxyphenyl)methyleneamino]propylamino}cyclopent-1-ene-1-dithiocarboxylato)nickel(II) has been determined by X-ray crystallography. The complex has a tetrahedrally distorted square-planar geometry, and spectroscopic results indicated that this structure is retained even in strong co-ordinating solvents. Moreover, as the spectroscopic properties of this complex are very similar to those of the other complexes studied, it can be concluded that the introduction of substituents in the ligand does not induce significant structural changes. Electrochemical and EPR data show that the complexes are typically reduced to four-co-ordinate nickel(I) species although, with some of the ligands, formation of six-co-ordinate nickel(I) complexes was observed and an explanation is put forward to account for these different behaviours. Cyclic voltammetry studies showed that these complexes may also be oxidized but chemically and/or electrochemically oxidized solutions did not show any evidence for the formation of nickel(III) species.