Mononuclear, distorted square planar [Ni(II)(ER)(P(o-C(6)H(4)S)(2)(o-C(6)H(4)SH))](-) (ER = SePh (1), 2-S-C(4)H(3)S (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-C(4)H(3)S)(3)](-) and P(o-C(6)H(4)SH)(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 nu(SH) stretching band (2273 and 2283 cm(-)(1) (KBr) for complexes 1 and 2, individually) and (1)H NMR spectra (delta 8.079 (d) (CD(2)Cl(2)) and 8.39 (d) (C(4)D(8)O) ppm (-SH) for complexes 1 and 2, respectively) and subsequently confirmed by X-ray diffraction study. The exo-thiol proton (o-C(6)H(4)SH) in complexes 1 and 2 was identified as a D(2)O exchangeable proton from NMR and IR studies and was quantitatively removed by Lewis base Et(3)N to yield Ni(II) dimer [Ni(II)(P(o-C(6)H(4)S)(3))](2)(2)(-) (5). Instead of the ligand-based oxidation to form dinuclear Ni(II) complexes and dichalcogenide, oxidation of THF-CH(3)CN solution of complexes 1 and 2 by O(2) resulted in the formation of the mononuclear, distorted trigonal bipyramidal [Ni(III)(ER)(P(o-C(6)H(4)S)(3))](-) (ER = SePh (3), 2-S-C(4)H(3)S (4)) accompanied by byproduct H(2)O identified by (1)H 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.