Kinetics of redox reactions between complexes of molybdenum and iron: the oxidation of iron(II) by molybdenum(VI) and of [Mo(H2O)6]3+ by [Fe(H2O)6]3+

Abstract

In 8 mol dm^–3 hydrochloric acid Mo^VI is reduced to Mo^V by Fe^II. Under these conditions, Mo^VI exists predominantly as MoO₂Cl₂ and Mo^V as MoOCl₅^2–. Spectrophotometric studies indicate that the stoicheiometry of the reaction is exactly 1:1 and that the equilibrium is far to the side of the products. Studies of the kinetics of this redox process by stopped-flow techniques revealed that the rate of reaction is first-order in each reactant and that the second-order rate constant is k=(3.6 ± 0.1)× 10³ dm³ mol^–1 s^–1(20 °C, 8 mol dm^–3 HCI). The mechanism of the reaction is not known, but a chloride-bridged inner-sphere process appers plausible. Equilibrium studies of the oxidation of [Mo(H₂O)₆]³⁺ by Fe³⁺ in 1 mol dm^–3p-toluenesulphonic acid (Hpts) indicate that a dimeric Mo^V species, Mo₂O₄²⁺, is the first stable product. With an excess of Fe³⁺, this species is oxidized to Mo^VI. In kinetic studies with Fe³⁺ in excess, three processes could be observed: (a) the disappearance of Mo³⁺, (b) the formation of Mo₂O₄²⁺, and (c) the disappearance of Mo₂O₄²⁺. Process (a) occurs in the ms range. It is described by the equation –d[Mo³⁺]/dt=k₁[Mo³⁺][Fe³⁺], with k₁=(1.30 ± 0.05)× 10³ dm³ mol^–1 s^–1(25 °C, 1 mol dm^–3 Hpts), and obviously proceeds by an outer-sphere mechanism. Steps (b) and (c) overlap strongly (seconds to minutes). The measured reaction curves for (b) and (c) can be satisfactorily described by two superimposed exponentials of rather similar time constants. The formation of Mo₂O₄²⁺ from the products of the fast step is discussed in terms of a mechanism which is in approximate though not complete agreement with the experimental data. The second-order rate constant for the oxidation of the intermediate Mo₂O₄²⁺ by Fe^III which has been evaluated from the reaction curves agrees well with that of a previous study in which Mo₂O₄²⁺ was oxidized directly.