Proton Transfer to Nickel−Thiolate Complexes. 1. Protonation of [Ni(SC<sub>6</sub>H<sub>4</sub>R-4)<sub>2</sub>(Ph<sub>2</sub>PCH<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>)] (R = Me, MeO, H, Cl, or NO<sub>2</sub>)
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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 ΔG⧧ = 13.6 ± 0.3 kcal mol-1 for all the derivatives but the values of ΔH⧧ and ΔS⧧ vary with R (R = MeO, ΔH⧧ = 8.5 kcal mol-1, ΔS⧧ = −16 cal K-1 mol-1; R = Me, ΔH⧧ = 10.8 kcal mol-1, ΔS⧧ = −9.5 cal K-1 mol-1; R = Cl, ΔH⧧ = 23.7 kcal mol-1, ΔS⧧ = +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.
