Reaction paths and kinetics in the Cr2+ reduction of the MoV2 and MoIV3 ions [Mo2O4(H2O)6]2+ and [Mo3O4(H2O)9]4+

Abstract

Inner-sphere paths are observed in the Cr²⁺ reduction of 10^–4–10^–3 M solutions of Mo^V₂, [Mo₂O₄(H₂O)₆]²⁺, and Mo^IV₃, [Mo₃O₄(H₂O)₉]⁴⁺, at 25 °C, I= 2.0 M [Na(pts)]. In 1.9 M Hpts (p-toluenesulphonic acid) and with [Cr²⁺] in > 10-fold excess the reaction of Mo₂^V proceeds via a grey-green Cr-containing intermediate (≈1 min), to give a product with the Mo^III₂ spectrum (≈24 h). The rate law for formation of the intermediate is of the form k₁[Cr²⁺]²[Mo^V₂][H ⁺], with k₁= 9.1 × 10³ M^–3 s^–1. The decay of the intermediate is independent of [Cr²⁺] and can be expressed as k₂[Intermediate][H⁺], with k₂=2.0 × 10^–5 M^–1 s^–1 believed to correspond to a process involving loss of Cr^III. With a 2: 1 Cr²⁺ : Mo^V₂ ratio of reactants, evidence for two intermediates is obtained (≈ 1 min), one (green in colour) giving a Cr : Mo ratio of 1 : 1, and the other the same as that generated in the reaction of excess of Cr²⁺ with Mo^V₂ giving a Cr : Mo ratio of 2:1. Some 30% of the Mo is separated as Mo^V₂. Over longer periods (≈22 h) 60–70% overall conversion to Mo^IV₃ is observed, and 24–30% of the Mo is present as Mo^V₂. In separate experiments, with Cr²⁺ in > 10-fold excess, Mo^IV₃ is reduced to Mo^III,III,IV in a two-stage process, complete within 1 min, and then through to Mo^III₃(≈40 min). Reduction of Mo^V₂ to either Mo^III₂ or Mo^III₃ is observed therefore, depending on whether a single addition of excess of Cr²⁺ or successive additions of Cr²⁺ are made.