ChemInform Abstract: Solution Chemistry of the Tungsten(V) Aqua Dimer W2O2+ 4: Kinetics of Oxidation with (IrCl6)2- and Other Oxidants and Conditions for Tungsten Blue Formation.

ChemInform Abstract

The kinetics of the oxidation of W₂O₄²⁺ (in excess) to W(VI) with (IrCl₆)^2- is studied spectrophotometrically in TosOH (I = 1.0 M TosLi). The rate law obtained is consistent with the reaction sequence shown in the scheme. The rate constant (6.6·10⁴ M^-1 s^-1 exhibits no (H+) dependence (0.5-1.0 M) and is ∼ 26 times greater than that observed for H₂O substitution by NCS- on W₂O₄²⁺. The reaction is therefore most likely an outer-sphere redox process. It is 6·10⁵ times greater than the corresponding Mo₂O₄²⁺ reaction, a trend that is observed also for the (IrCl₆)^2- oxidation of the edta complexes (W2O4(edta))2- and (Mo2O4(edta))2- (∼ 105) and the (PtCl₆)^2- and BrO₃^- oxidations of W₂O₄²⁺ and Mo₂O₄²⁺. These observations are consistent with W₂O₄²⁺ being a much more powerful reductant than Mo₂O₄²⁺. The blue color of W(V)/W(VI) mixed-valence species, which is developed in all cases with W₂O₄²⁺ in excess, is generated by excess W(V)₂ reacting with W(VI). In corresponding experiments no molybdenum blue mixed-valence species are formed.