The protonation of [Ni(SC6H4R-4)(triphos)](+) (triphos = PhP{CH2CH2PPh2}(2); R = NO2, Cl, H, Me, or MeO) by [lutH](+) (lut = 2,6-dimethylpyridine) to form [Ni(S(H)C6H4R-4)(triphos)](2+) is an equilibrium reaction in MeCN. Kinetic studies, using stopped-flow spectrophotometry, reveal that the reactions occur by a two-step mechanism. Initially, [lutH](+) rapidly binds to the complex (K-2(R)) in an interaction which probably involves hydrogen-bonding of the acid to the sulfur. Subsequent intramolecular proton transfer from [lutH](+) to sulfur (k(3)(R)) is slow because of both electronic and steric factors. The X-ray crystal structures of [Ni(SC6H4R-4)(triphos)](+) (R = NO2, H, Me, or MeO) show that all are best described as square-planar complexes, with the phenyl substituents of the triphos ligand presenting an appreciable barrier to the approach of the sterically demanding [lutH](+) to the sulfur. The kinetic characteristics of the intramolecular proton transfer from [lutH](+) to sulfur have been investigated. The rate of intramolecular proton transfer exhibits a nonlinear dependence on Hammett sigma(+), with both electron-releasing and electron-withdrawing 4-R-substituents on the coordinated thiolate facilitating the rate of proton transfer (NO2 > Cl > H > Me < MeO). The rate constants for intramolecular proton transfer correlate well with the calculated electron dnsity of the sulfur. The temperature dependence of the rate of the intramolecular proton transfer reactions shows that DeltaH(double dagger) is small but increases as the 4-R-substituent becomes more electron-withdrawing {DeltaH(double dagger) = 4.1 (MeO), 6.9 (Me), 11.4 kcal mol(-1) (NO2)}, while DeltaS(double dagger) becomes progressively less negative {DeltaS(double dagger) = -50.1 (MeO), -41.2 (Me), -16.4 (NO2) cal K-1 mol(-1)}. Studies with [lutD](+) show that the rate of intramolecular proton transfer varies with the 4-R-substituent {(k(3)(NO2))(H)/(k(3)(NO2))(D) = 0.39; (k(3)(Cl))(H)/(k(3)(Cl))(D) = 0.88; (k(3)(Me))(H)/(k(3)(Me))(D) = 1.3; (k(3)(MeO))(H)/(k(3)(MeO))(D) = 1.2}.