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+
In 8 mol dm–3 hydrochloric acid MoVI is reduced to MoV by FeII. Under these conditions, MoVI exists predominantly as MoO2Cl2 and MoV as MoOCl52–. 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 MoV species, Mo2O42+, is the first stable product. With an excess of Fe3+, this species is oxidized to MoVI. 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 FeIII which has been evaluated from the reaction curves agrees well with that of a previous study in which Mo2O42+ was oxidized directly.