The kinetics of the equilibrium reaction between [Ni(SC6H4R-4)(2)(dppe)] (R = MeO, Me, H, Cl, or NO2; dppe = Ph2PCH2CH2PPh2) and mixtures of [lutH](+) and lut (lut = 2,6-dimethylpyridine) in MeCN to form [Ni(SHC6H4R-4)(SC6H4R-4)(dppe)](+) have been studied using stopped-flow spectrophotometry. The kinetics for the reactions with R = MeO, Me, H, or Cl are consistent with a single-step equilibrium reaction. Investigation of the temperature dependence of the reactions shows that DeltaG(double dagger) = 13.6 +/- 0.3 kcal mol(-1) for all the derivatives but the values of DeltaH(double dagger) and DeltaS(double dagger) vary with R (R = MeO, DeltaH(double dagger) = 8.5 kcal mol(-1), DeltaS(double dagger) = -16 cal K-1 mol(-1); R = Me, DeltaH(double dagger) = 10.8 kcal mol(-1), DeltaS(double dagger) = -9.5 cal K-1 mol(-1); R = Cl, DeltaH(double dagger) = 23.7 kcal mol(-1), DeltaS(double dagger) = +33 cal K-1 mol(-1)). With [Ni(SC6H4NO2-4)(2)(dppe)] a more complicated rate law is observed consistent with a mechanism in which initial hydrogen-bonding of [lutH](+) to the complex precedes intramolecular proton transfer, It seems likely that all the derivatives operate by this mechanism, but only with R = NO2 (the most electron-withdrawing substituent) does the intramolecular proton transfer step become sufficiently slow to result in the change in kinetics. Studies with [lutD]+ show that the rates of proton transfer to [Ni(SC6H4R-4)(2)(dppe)] (R = Me or Cl) are associated with negligible kinetic isotope effect. The possible reasons for this are discussed. The rates of proton transfer to [Ni(SC6H4R-4)(2)(dppe)] vary with the 4-R-substituent, and the Hammett plot is markedly nonlinear. This unusual behavior is attributable to the electronic influence of R which affects the electron density at the sulfur.