Chromium(VI), chromium(V) and chromium(IV) oxidation of 12-tungstocobaltate(II)

Abstract

The reaction between CrVI and 12-tungstocobaltate(II) was carried out in 2.0 mol dm−3 HCl and followed a simple second order rate law. The reaction was catalysed by hydrogen ion due to the formation of active H2CrO4 and was inhibited by chloride ion as, in its presence, conversion of the active species into inactive chlorochromate occurs. Chromium(V) and chromium(IV) were generated in situ by the use of CrVI—VIV or CrVI—2-ethyl-2-hydroxybutyric acid and CrVI—i-PrOH reactions respectively, and the oxidation of 12-tungstocobaltate(II) by these atypical oxidation states, was also studied. The rate constants for the oxidation of 12-tungstocobaltate(II) by CrVI, CrV and CrIV were found to be in the ratio 1:1.2:5.2 respectively. The ionic strength did not affect the reaction, while decrease in the solvent polarity increased the rate of the reaction. The activation parameters were also determined and the values ΔH‡, ΔG‡ and ΔS‡ were found to be 52.4 ± 6 kJ mol−1, 100.8 ± 7 kJ mol−1, −151.7 ± 10 J K−1 mol−1 respectively, supporting the mechanism proposed.