The shift of the IR nu(S-H) frequency to lower wavenumbers for the series of complexes [Ni-II(L)(P-(o-C6H4S)(2)(o-C6H4SH))](0/1-) (L = PPh3 (1), Cl (6), Se-p-C6H4-Cl (5), S-C4H3S (7), SePh (4)) indicates that a trend of increasing electronic donation of the L ligands coordinated to the Ni(II) center promotes intramolecular [Ni-S center dot center dot center dot H-S] interactions. Compared to the Ni center dot center dot center dot S(H) distance, in the range of 3.609-3.802 angstrom in complexes 1 and 4-7, the Ni center dot center dot center dot S(CH3) distances of 2.540 and 2,914 angstrom observed in the [Ni-II(PPh3)(P(o-C6H4S)(2)(o-C6H4-SCH3))] complexes (8a and 8b, two conformational isomers with the chemical shift of the thioether methyl group at delta 1.820 (-60 degrees C) and 2.109 ppm (60 degrees C) (C4D8O)) and the Ni center dot center dot center dot S(CH3) distances of 3.258 and 3.229 angstrom found in the [Ni-II(L)(P(o-C6H4S)(2)(o-C6H4SCH3))](1-) complexes (L = SPh (9), SePh (10)) also support the idea that the pendant thiol protons of the Ni(II)thiol complexes 1/4-7 were attracted by both the sulfur of thiolate and the nickel. The increased basicity (electronic density) of the nickel center regulated by the monodentate ligand attracted the proton of the pendant thiol effectively and caused the weaker S center dot center dot center dot H bond. In addition, the pendant thiol interaction modes in the solid state (complexes 1a and 1b, Scheme 1) may be controlled by the solvent of crystallization. Compared to complex 1a, the stronger intramolecular [Ni-S center dot center dot center dot H-S] interaction (or a combination of [Ni-S center dot center dot center dot H-S]/[Ni center dot center dot center dot H-S] interactions) found in complexes 4-7 led to the weaker S-H bond strength and accelerated the oxidation (by O-2) of complexes 4-7 to produce the [Ni-Y(L)(P(o-C6H4S)(3))](1-) (L = Se-p-C6H4-Cl (11), SePh (12), S-C4H3S (13)) complexes.