The mechanism of the photochemical production of hydrogen from aqueous solutions of hydridotris(triethylphosphine)platinum(II)

Abstract

The photolysis of aqueous solutions of [PtH(PEt₃)₃]⁺ at pH 2–12 in the presence of SO₄^2– with near-u.v. light produces hydrogen and [Pt(PEt₃)₃(H₂O)]²⁺. In the presence of Cl^–, hydrogen, [PtCl(PEt₃)₃]⁺, and [PtCl₂(PEt₃)₂] are obtained. Variations in the amount of added chloride lead to the rate expression d(H₂)/dt=a[Cl^–]/(b+c[Cl^–]²) which can be accommodated by the formation of intermediates, [PtH₂(PEt₃)₃Cl]⁺ and [PtH₂(PEt₃)₃Cl]Cl, the latter being present as a ‘tight’ ion pair, if loss of hydrogen only occurs from the former. The first-order dependence on light intensity is taken as evidence that the platinum(IV) dihydrides are formed by protonation of the ground state of [PtH(PEt₃)₃]⁺. A similar mechanism operates in the presence of SO₄^2– except that photochemical hydrogen production occurs more readily from [PtH₂(PEt₃)₃(H₂O)]²⁺ than from [PtH₂(PEt₃)₃(SO₄)].