Electrostriction and counter ion effects in an outer-sphere electron transfer reaction. Kinetics of the reduction of hexachloroiridate(IV) by iodide ion

Abstract

The kinetics of the reaction between the hexachloroiridate(IV) ion and iodide ion in aqueous acidic media has been monitored spectrophotometrically in the temperature range 20 to 40 °C, at ambient and at elevated pressures (up to 125 MPa). With suitable selection of reactant concentrations the reaction can be studied as simple pseudo first-order (iodide ion in excess) yielding a second-order rate constant of 1.42 × 10³ M⁻¹ s⁻¹ at 25 °C and atmospheric pressure. This compares favourably with a value of 1.38 × 10³ M⁻¹ s⁻¹ for the same parameter, determined previously under the same conditions. The reaction rate varies to a limited degree with variation in the conjugate base of the acids employed, which may be the result of a secondary medium effect. Potassium ions exert a significant catalytic effect, a finding that parallels those reported for other anion–anion redox reactions. The reaction is moderately sensitive to temperature: ΔH^‡ ranges from 18 to 34 kJ mol⁻¹ depending on the reaction medium. All ΔS^‡ values are distinctly negative, suggesting a degree of molecular ordering and/or increasing electrostriction in the rate limiting step. Under all conditions employed pressure accelerates the reaction rate, resulting in large negative volumes of activation, typically −20 to −25 cm³ mol⁻¹. How the thermal and activation parameters relate to the mechanism is discussed.