Kinetics and mechanism of the reduction of dodecatungstocobaltate(III) by oxalate. The catalytic role of alkali metal ions through outer-sphere bridging

Abstract

The kinetics of reduction of dodecatungstocobaltate(III) by oxalate follow the rate laws k_obs.= 2k₁[HC₂O₄^–] and k_obs.= 2k₂[C₂O₄^2–] at high and low hydrogen ion concentrations respectively. Both these apparent second-order rate constants (k₁ and k₂) depend on the concentration and type of cation present; k₁ and k₂ decrease in the order K⁺ > Na⁺∼ Li⁺ and K⁺ > Na⁺ > Li⁺ respectively. The K⁺ and Na⁺ assisted paths have been studied in detail. The rate constants for these catalysed paths along with those for uncatalysed ones have been evaluated. Thermodynamic parameters (ΔH^‡ and ΔS^‡) associated with k₁ and k₂ have been determined. A cation-bridged outer sphere electron transfer mechanism is most likely to take place, with simultaneous generation of free radicals (C₂O₄˙^–).