The accessibility of the +3 oxidation state for nickel(II) complexes with N3O2 pentadentate Schiff base ligands prepared by condensation of naphthaldehyde or 3,5-dichlorosalicylaldehyde with triamines (Scheme 1) has been assessed in several solvents, by combining cyclic voltammetry and EPR spectroscopy of the oxidised metal complexes. The data show that trien/Metrien-based ligands act in pentadentate fashion and allow for the stabilisation of electrochemically and chemically generated nickel(III) complexes in all solvents used. The complexes, formulated as [NiIIIL(solv)]+, are low-spin and have a 2A1, [a dz + b dxy], ground state, with a ⩾ b. For dien-based ligands, steric constraints prevent coordination of the amine nitrogen atom of the pentadentate ligand to the metal centre, thus enforcing a fourfold coordination on the ligand. The resulting complexes can be oxidised to nickel(III) species, but only when using iodine as oxidant, in DMF or (CH3)2SO. The complexes are formulated as [NiIIIL(solv)2]+, and are low-spin and have a 2A1, [a dz + b dxy], ground state, with a >> b. The binding of pyridine and cyanide ion to NiIII trien/Metrien-based complexes was studied by EPR. The data obtained show that these polydentate ligands provide a flexible coordination sphere, and that by varying the ratio equatorial/axial ligand field it is possible to obtain NiIII in three different ground states. For pyridine adducts, the same ground state as the parent complex was observed with Metrien-based ligands, while for trien-based ligands an inverted ground state (2A1, [a dz + b dxy], with a << b) was observed, as the latter provides a higher axial ligand field. On the other hand, coordination of cyanide adducts induces a change in the principal molecular axis of the NiIII complexes, in which the cyanide ions occupy equatorial positions, and in which a 2A1, [a dz + b dxy], ground state, with a >> b, is observed.