Kinetic studies on the oxidation of hexa-aquamolybdenum(III) with [IrCl6]2–, [Co(C2O4)3]3–, and aquavanadium(IV)

Abstract

Three one-equivalent reagents [IrCl₆]^2–, [Co(C₂O₄)₃]^3–, and VO²⁺ oxidise hexa-aquamolybdenum(III), according to equations, for example, 2Mo^III+ 4Ir^IV→ Mo^V₂+ 4Ir^III, with the aquamolybdenum(V) dimer, Mo₂O₄²⁺, the sole molybdenum product. Rate laws are consistent with a rate-determining first stage, e.g. Mo^III+ Ir^IV→ Mo^IV+ Ir^III, in which Mo^IV is formed. No build-up of Mo^IV was detected. Reactions were studied with the molybdenum(III) reactant in excess. With [IrCl₆]^2–(reduction potential 0.89 V) first-order rate constants, k_obs., from the (stopped-flow) decay of [IrCl₆]^2– at 489 nm gave a dependence k_obs./2[Mo^III]=k_a+k_b[H⁺]^–1, where at 25 °C, I= 0.2 M [Li(pts)], k_a= 3.4 × 10⁴ M^–1s^–1 and k_b= 2.9 × 10⁴ s^–1. On replacing p-toluenesulphonate (pts^–) by trifluoromethanesulphonate (tfms^–)k_obs. showed little variation (ca. 12%). The oxidant absorbance decay was also monitored in the [Co(C₂O₄)₃]^3–(0.58 V) and VO²⁺(aq)(0.36 V) reactions (t_½ > 1 min). At 25 °C, I= 2.0 M [Li(pts)] the method of initial rates (R) gave dependences R_Co= 2k_c[Mo^III][Co^III]+ 4k_d[Mo^III]²+ 2k_e[Mo^III] for [Co(C₂O₄)₃]^3–(k_c= 0.34 M^–1s^–1), and R_v= 2k_f[Mo^III][V^IV]+ 4k_d[Mo^III]²+ 2k_e[Mo^III] for VO²⁺(k_f= 3.3 × 10^–3 M^–1s^–1). On replacing pts^– by tfms^– the oxidant-independent terms k_d(1.87 × 10^–3 M^–1 s^–1) and k_e(4.9 × 10^–5 s^–1), which are common to both reactions, make no contribution. It is concluded that pts^– complexes to aquamolybdenum(III). Rate constants for the [IrCl₆]^2– reaction are much faster than those obtained for substitution into the molybdenum(III) co-ordination sphere, and an outer-sphere mechanism is strongly implied. The [Co(C₂O₄)₃]^3– oxidation (k_c) is independent of [H⁺] and exhibits behaviour consistent with an inner-sphere substitution-controlled process.