Mononuclear, distorted square planar [Ni-II(ER)(P(o-C6H4S)(2)(o-C6H4SH))](-) (ER = SePh (1), 2-S-C4H3S (2)) with a S-H proton directly interacting with both nickel and sulfur atoms were prepared by reaction of [Ni(CO)(SePh)(3)](-)/[Ni(CO)(2-S-C4H3S)(3)](-) and P(o-C6H4SH)(3), individually. The presence of combinations of intramolecular [Ni-S...H-SR]/[Ni...H-SR] interactions was verified in the solid state by the observation of an IR v(SH) stretching band (2273 and 2283 cm(-1) (KBr) for complexes 1 and 2, individually) and H-1 NMR spectra (delta 8.079 (d) (CD2Cl2) and 8.39 (d) (C4D8O) ppm (-SH) for complexes 1 and 2, respectively) and subsequently confirmed by X-ray diffraction study. The exo-thiol proton (o-C6H4SH) in complexes 1 and 2 was identified as a D2O exchangeable proton from NMR and IR studies and was quantitatively removed by Lewis base Et3N to yield Ni(II) dinner [Ni-II(P(o-C6H4S)(3))](2)(2-) (5). Instead of the ligand-based oxidation to form dinuclear Ni(II) complexes and dichalcogenide, oxidation of THF-CH3CN solution of complexes 1 and 2 by O-2 resulted in the formation of the mononuclear, distorted trigonal bipyramidal [Ni-III(ER)(P(o-C6H4S)(3))](-) (ER = SePh (3), 2-S-C4H3S (4)) accompanied by byproduct H2O identified by H-1 NMR, respectively. The 4.2 K EPR spectra of complexes 3 and 4 exhibiting high rhombicities with three principal g values of 2.304, 2.091, and 2.0 are consonant with Ni(III) with the odd electron in the d(z)(2) orbital. Complex 3 undergoes a reversible Ni-III/II process at E-1/2 = -0.67 V vs Ag/AgCl in MeCN.