A revisit to the oxidation of the 12-tungstocobaltate(II) ion by permanganate ion and comparison with the oxidation of octacyanomolybdate (IV) ion

Abstract

The oxidation of 12-tungstocobaltate(<b>II</b>) ion, [Co^IIW]^6- , by MnO₄^- ion in HClO₄ has the stoichiometric ratio Δ [Co^IIW]^6-]/Δ [MnO₄^-]= 5.0-+ 0.1. The reaction under pseudo-first order conditions ([Co^IIW]^6- >> [MnO₄^-]) is first order

each in [MnO₄^-) and [Co^IIW^6-] ions. There is spectrophotometric evidence for a complex formation between permanganate and [Co^IIW]^6-ion both in aqueous and acidic media with the equilibrium constant in the acidic medium(<i>K</i>^h = 465 dm³ mol^-1) being greater than that in aqueous solution (<i>K</i>^o = 156 dm³ mol^-1). The ion pair formation between alkali metal ion and [Co^IIW]^6-is substantiated by the increase in the optical density with the increase in [Na⁺] and the estimated value of the equilibrium constant is <i>K </i>= 586 ± 16 dm³ mol^-1. The rate increases with increase in [H⁺] and the empirical rate equation, rate<i>= a + b</i> [H⁺], indicates that both MnO₄^- and HMnO₄ are the reactive entities because [Co^IIW]^6- ion does not get protonated. The <i>k</i>_H/<i>k</i> ratio at 25 °C is 4.72 ×10⁴ which is in very good agreement with the corresponding ratio found in the oxidations of [Mo(CN)₈]^1- and [Fe(phen)₃]²⁺ ions. There is no spectral evidence for long-lived Mn⁴⁺ or Mn³⁺ ions. A linear correlation between the rate and the polarisability, α_M⁺, of the alkali metal ion suggests that these ions act as effective bridg between the

oppositely charged ions. The outer-sphere nature of the reaction is supported by the application of the Marcus cross relation.