Kinetics of the one-equivalent reactions of vanadium(II), vanadium(III), and iron(II) with hexachloroiridate(IV)

Abstract

Vanadium(II) reduces hexachloroiridate(IV) at a rate which is too fast to follow (k > 4 × 10⁶ l. mole^–1 sec.^–1 at 25°) by use of the stopped-flow technique. The reaction of vanadium(III) with hexachloroiridate(IV) is considerably slower and can be studied by both conventional and stopped-flow techniques. The rate law is of the form d[V^IV] dt=–d[IrCl₆^2–]/dt=k′[V^III][IrCl₆^2–][H⁺]^–1, and at 25° and µ= 1·0M(LiClO₄), k′= 52·2 sec.^–1, ΔH‡= 12·4 (±0·5) kcal. mole^–1 and ΔS‡=–9·0 (±1·0) e.u. Runs in which a large excess of vanadium(III) was used did not provide kinetic evidence for ion-pair formation. The reaction of iron(II) follows the rate law –d[IrCl₆^2–]/dt=k[Fe^II][IrCl₆^2–]. At 25° and µ= 1·0M(LiClO₄), k= 3·0 × 10⁶ l. mole^–1 sec.^–1, ΔH‡= 2·3 (±0·3) kcal. mole^–1, and ΔS‡=–28 (±4) e.u. All three reactions are believed to be of the outer-sphere type.