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

The oxidation of 12-tungstocobaltate(II) ion, [(CoW)-W-II](6-), by MnO4- io n in HClO4 has the stoichiometric ratio Delta [(CoW6-)-W-II]/Delta [MnO4-] = 5.0 +/- 0.1. The reaction under pseudo-first order conditions ((CoW)-W-II ](6-) > > [MnO4-]) is first order each in [MnO4-] and [(CoW6-)-W-II] ions. There is spectrophotometric evidence for a complex formation between perman ganate and [(CoW)-W-II](6-) ion both in aqueous and acidic media with the e quilibrium constant in the acidic medium(K-h = 465 dm-1 mol(-1)) being grea ter than that in aqueous solution (K-o = 156 dm(3) mol-1). The ion pair for mation between alkali metal ion and [(CoW)-W-II](6-) is substantiated by th e increase in the optical density with the increase in [Na+] and the estima ted value of the equilibrium constant is K = 586 +/- 16 dm(3) mol(-1). The rate increases with increase in [H+] and the empirical rate equation, rate = a + b[H+], indicates that both MnO4- and HMnO4 are the reactive entities because [(CoW)-W-II](6-) ion does not get protonated. The k(H)/k ratio at 2 5 degreesC is 4.72 X 10(4) which is in very good agreement with the corresp onding ratio found in the oxidations of [Mo(CN)(8)](4-) 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, alpha (+)(M). o f the alkali metal ion suggests that these ions act as effective bridg betw een the oppositely charged ions. The outer-sphere nature of the reaction is supported by the application of the Marcus cross relation.