Kinetics and mechanism of the inner-sphere reduction of hexachloroplatinate(IV) by dithionite in acetate buffer

Abstract

The kinetics of reduction of hexachloroplatinate(IV) by dithionite have been examined spectrophotometrically in sodium acetate–acetic acid buffer medium in the temperature range 288–303 K. The reaction is first order in both platinum(IV) species and dithionite. H⁺ ion has an inhibiting effect on the rate in the pH range 3.68–4.80. The pseudo-first order rate constant increased upon increasing both ionic strength and dielectric constant. The suggested mechanism involves an initial transition state between two like charged ions, which then decomposes to give SO₃ ²⁻ through the intermediate formation of free radicals. The presence of free radicals was confirmed by performing the reaction in the presence of acrylamide. PtCl₆ ²⁻ is finally reduced to PtCl₄ ²⁻, as confirmed by thermogravimetric analysis and IR spectrophotometry. The values of ∆H^≠ and ∆S^≠ associated with the rate-determining step have been calculated as 33 ± 4 kJ mol⁻¹ and −141 ± 7 JK mol⁻¹, respectively. The values of ∆H° and ∆S° for the dissociation of HS₂O₄ ⁻ are 16 ± 4 kJ mol⁻¹ and −14 ± 7 JK mol⁻¹, respectively.