Kinetic studies on the oxidation of trimeric aquomolybdenum(IV)

Article abstract of DOI:10.1021/ic50226a025

Oxidations of the Mo(IV) aquo trimer, Mo₃O₄⁴⁺, with IrCl₆^2- and Fe(phen)₃³⁺ have been studied under the conditions [H⁺] = 0.6-2.0 M and I = 2.0 M (LiClO₄). With Mo^IV₃ in large excess, the IrCl₆^2- reaction can be expressed as 2Mo^IV₃ + 6Ir^IV → 3Mo^V₂ + 6Ir^III, and the initial step Mo^IV₃ + IrCl₆^2- is rate determining. Second-order rate constants k_Ir are dependent on [H⁺]: k_Ir = k_-1[H⁺]^-1, where at 25°C k_-1 = 1.36 s^-1, ΔH? = 14.1 kcal mol^-1, and ΔS? = -9.6 cal K^-1 mol^-1. An alternative stoichiometry Mo^IV₃ + 4Ir^IV → Mo^V₂ + Mo^VI + 4Ir^III cannot be entirely ruled out, and at nearly equivalent amounts of reactants the slower oxidation of Mo^V₂ to Mo^VI contributes. With Fe(phen)₃³⁺ a different reactivity pattern is observed in that oxidation of Mo^V₂ is more rapid than that of Mo^IV₃, and the equation Mo^IV₃ + 6Fe(phen)₃³⁺ → 3Mo^VI + 6Fe(phen)₃²⁺ applies. This different behavior is accounted for by the reaction of Fe(phen)₃³⁺ occurring exclusively by an outer-sphere mechanism, whereas IrCl₆^2- can react inner sphere. The initial step, Mo^IV₃ + Fe(phen)₃³⁺, is again rate determining and at 25°C k_-1 for the [H⁺]^-1-dependent path is 0.53 s^-1, I = 2.0 M. No evidence was obtained for stable trimeric mixed-oxidation-state species as product(s) of the oxidation of Mo^IV₃. The complex Co(C₂O₄)₃^3- does not oxidize Mo^IV₃, whereas (as reported elsewhere) oxidation of aquo Mo^III and Mo^III₂ through to Mo^V₂ is rapid. It is concluded that oxidation of monomeric and/or dimeric Mo(IV) occurs more readily than that of the trimer. No oxidation of Mo^IV₃ by the two-electron oxidant PtCl₆^2- is observed.