CERIUM(4) OXIDATIONS OF TRIS(OXALATO)CHROMATE(3) AND TRIS(OXALATO)RHODATE(3) IONS IN AQUEOUS SULFURIC ACID

Cr(C2O4)3-3 and Rh(C2O4)3-3 react with Ce(IV) in aqueous sulfuric acid with the stepwise oxidation of coordinated oxalate, the initial reactions proceeding to produce bis-oxalato complexes. For the Cr(C2O4)3-3-Ce(IV) system evaluation of rate constants for the initial direct redox reaction requires consideration of the competing aquation of Cr(C2O4)3-3. In 1 M sulfuric acid at 25° the second-order rate constant for the direct one-electron oxidation of Cr(C2O4)3-3 is 4.0 (±0.5) × 10-2 M-1 sec-1, while for the corresponding direct one-electron oxidation of Rh(C2O4)3-3 the rate constant is 6.1 (±1.0) × 10-4 M-1 sec-1. Rate behavior for several aqueous-acidic sulfate solvents, differing in concentrations of hydrogen ion and sulfate ion, suggests that activated complexes for the initial direct redox reactions of Cr(C2O4)3-3 and Rh(C2O4)3-3 are of generally similar but not identical compositions. The measured ΔH‡ values for both systems exhibit slight temperature dependences which are probably caused by small changes with temperature in the average compositions of activated complexes. At corresponding temperatures the ΔH‡ value for the rhodium system is significantly higher than that for the chromium system. The initial direct redox step for each complex is interpreted as involving oxidation of a coordinated oxalate to an oxalate radical anion. Factors which are likely to contribute to differences in the redox reactivities of Rh(C2O4)3-3 and Cr(C2O4)3-3 are discussed. © 1969, American Chemical Society. All rights reserved.