REACTIONS OF PENTACYANOCOBALTATE(2) WITH HYDROGEN PEROXIDE HYDROXYLAMINE AND CYANOGEN IODIDE

The oxidation of pentacyanocobaltate(H) by several molecules, namely hydrogen peroxide, hydroxylamine, and cyanogen iodide, was examined in aqueous solution. The reaction in each case conforms to the stoichiometry 2Co(CN)53- + X-Y → Co(CN)5X3- + Co(CN)5Y3-, where X-Y = HO-OH, HO-NH2, or I-CN. Each of the reactions exhibits second-order kinetics according to the rate law – d[X-Y]/dr = k0[Co(CN)53-][X-Y], The values of k at 25° (M-1 sec-1), of ΔH± (kcal/mole), and of ΔS‡ (eu), respectively, were determined to be as follows: for HO-OH, 7.4 X 102, 4.2, -31; for HO-NH2, 5.3 X 10-1, 10.3, -35; for I-CN, 9.5, 12.7, -12. The results are interpreted in terms of free-radical mechanisms of the type: Co(CN)53- + X-Y → Co(CN)5X3- + Y- (rate determining); Co(CN)53- + Y→ Co(CN)5Y3-. Added I- interferes with the reaction between Co(CN)53- and H2O2 by reacting with the OH radical intermediate according to I- +- OH-→ I- + OH-, with subsequent capture of the I atom by Co(CN)53- to form Co(CN)5I3-. In the presence of added I- the stoichiometry thus becomes 2Co(CN)53- + H2O2 + I- → Co(CN)5OH3- + Co(CN)5I3- + OH-. © 1969, American Chemical Society. All rights reserved.