Kinetic Evidence for Intramolecular Proton Transfer Between Nickel and Coordinated Thiolate

The complexes [Ni(YR)(triphos)]BPh4 {Y = S, R = Ph or Et or Y = Se, R = Ph; triphos = (Ph2PCH2CH2)2PPh}
have been prepared and characterized, and the X-ray crystal structure of [Ni(SPh)(triphos)]BPh4 has been solved.
In MeCN, [Ni(YR)(triphos)]+ are protonated by [lutH]+ (lut = 2,6-dimethylpyridine) to give [Ni(YHR)(triphos)]2+. Studies
on the kinetics of these equilibrium reactions reveal an unexpected difference in the reactivities of [Ni(SPh)(triphos)]+
and [Ni(SEt)(triphos)]+. In both cases, the reactions exhibit a first-order dependence on the concentration of complex.
When R = Ph, the dependence on the concentrations of [lutH+] and lut is given by kobs = k1Ph[lutH+] + k-1Ph[lut],
which is typical of an equilibrium reaction where k1Ph and k-1Ph correspond to the forward and back reactions,
respectively. Analogous behavior is observed for [Ni(SePh)(triphos)]+. However, for [Ni(SEt)(triphos)]+, the kinetics
are more complicated , and kobs = {k1k2[lutH+] + (k-2 + k2)}/(k1[lutH+] + k-1[lut]), which is indicative of a mechanism
involving two coupled equilibria in which the initial protonation of the thiolate is followed by a unimolecular equilibrium
reaction that is assumed to involve the formation of an η2-EtS−H ligand. The difference in reactivity between the
complexes with alkyl and aryl thiolate ligands is a consequence of the {Ni(triphos)}2+ site “leveling” the basicities
of these ligands. The pKa's of the PhSH and EtSH constituents coordinated to the {Ni(triphos)}2+ are 16.0 and
14.6, respectively, whereas the difference in pKa's of free PhSH and EtSH differ by ca. 4 units. The pKa of [Ni(SeHPh)(triphos)]+ is 14.4. The more strongly σ-donating EtS ligand makes the {Ni(triphos)}2+ core sufficiently
electron-rich that the basicities of the sulfur and nickel in [Ni(SEt)(triphos)]+ are very similar; therefore, the proton
serves as a bridge between the two sites. The relevance of these observations to the proposed mechanisms of
nickel-based hydrogenases is discussed.