KINETICS AND MECHANISMS OF THE REDOX REACTIONS OF THE HYDROPEROXOCHROMIUM(III) ION

The reactions of the hydroperoxochromium(III) ion, (H2O)5CrO2H2+ (CrO2H2+), with Fe2+, VO2+, V2+, Cu+, Ti3+, Co([14]aneN4)2+, Co(Me6[14]aneN4)2+, Co(tim)2+, and [Ru(NH3)6]2+ have been studied in acidic aqueous solution. The reactions are accompanied by large negative entropies of activation, -110 J mol-1 K-1 for Fe2+ and -85 J mol-1 K-1 for Ti3+. All the reactions studied follow an isokinetic relationship in that DELTAH(double dagger) is a linear function of DELTAS(double dagger). The same is true for the analogous reactions of H2O2. It is proposed that the reactions of CrO2H2+ take place by an inner-sphere, Fenton-type process yielding pentaaquaoxochromium(IV), (H2O)5CrO2+ (CrO2+), as an intermediate. The reactivity of CrO2H2+ as an oxygen transfer reagent is about 20 times greater than that of H2O2. For example, the reactions with (en)2CoSCH2CH2NH22+ to yield (en)2CoS(O)CH2CH2NH2(2+) have rate constants 20.5 +/- 0.4 M-1 s-1 (CrO2H2+) and 1.36 M-1 s-1 (H2O2), both in 0.1 M HClO4 at 25-degrees-C. The chromyl ion, CrO2+, oxidizes CrO2H2+ to CrO22+ with a rate constant of (1.34 +/- 0.06) X 10(3) M-1 s-1 in 0.10 M HClO4 in H2O and 266 +/- 10 M-1 s-1 in D2O.