The reaction of three different 1-phenyl and 1,4-diphenyl substituted S-methylisothiosemicarbazides, H-2[L1-6], with Ni(OAC)(2)• 4H(2)O in ethanol in the presence of air yields six four-coordinate species [Ni(L1-6•)(2)] (1-6) where (L1-6•)(1-)represent the monoanionic T-radical forms. The crystal structures of the nickel complexes with 1-phenyl derivatives as in 1 reveal a square planar structure trans-[Ni(L1-3•)(2)], whereas the corresponding 1,4-diphenyl derivatives are distorted tetrahedral as is demonstrated by X-ray crystallography of [Ni(L-5•)(2)] (5) and [Ni(L-6•)(2)] (6). Both series of mononuclear complexes possess a diamagnetic ground state. The electronic structures of both series have been elucidated experimentally (electronic spectra magnetization data). The square planar complexes 1-3 consist of a diamagnetic central Ni(II) ion and two strongly antiferromagnetically coupled ligand π-radicals as has been deduced from correlated ab initio calculations; they are singlet diradicals. The tetrahedral complexes 4-6 consist of a paramagnetic high-spin Ni-II ion (S-Ni = 1), which is strongly antiferromagnetically coupled to two ligand. π-radicals. This is clearly revealed by DFT and correlated ab initio calculations. Electrochemically, complexes 1-6 can be reduced to form stable, paramagnetic monoanions [1-6](-) (S = 1/2). The anions [1-3]- are square planar Ni(II) (d,(8) SNi = 0) species where the excess electron is delocalized over both ligands (class 111, ligand mixed valency). In contrast, one-electron reduction of 4, 5, and 6 yields paramagnetic tetrahedral monoanions (S = 1/2). X-band EPR spectroscopy shows that there are two different isomers A and B of each monoanion present in solution. In these anions, the excess electron is localized on one ligand [Ni-II(L (4-6)•) (L4-6)]- where (L4-6)(2-) is the closed shell dianion of the ligands H2[L4-6] as was deduced from their electronic spectra and broken symmetry DFT calculations. Oxidation of 1 and 5 with excess iodine yields octahedral complexes [Ni-II(L-1,L-ox)(2)I-2] (7), [Ni-II(L-1,L-ox)(3)](I-3)(2) (8), and trans-[Ni-II(L-5,L-ox)(2)(I-3)(2)] (9), which have been characterized by X-ray crystallography; (L-1-6,L-ox) represent the neutral, two-electron oxidized forms of the corresponding dianions (L1 -6)(2-). The room-temperature structures of complexes 1, 5, and 7 have been described previously in refs 1-5.