Evidence for a Bond Homolysis Pathway for O2 Release in the Decomposition of Superoxochromium(III) Ion CrO22+

Article abstract of DOI:10.1021/ja00249a021

The cation (H2O)5CrO2²⁺ has a considerable lifetime in aqueous solution in the absence of Cr²⁺.One pathway for decomposition consists of bond homolysis, CrO2²⁺ -> Cr²⁺ + O2, with k = 2.5 x 10E-4 s^-1 at 25.0 deg C (ΔH^<*> = 25.1 +/- 1.1 kcal mol^-1, ΔS^<*> = 9.7 +/- 3.6 cal mol^-1 K^-1).The resulting Cr²⁺ then reacts very rapidly with CrO2²⁺, with k = 8 x 10E8 M^-1 s^-1, estimated by kinetic modeling of the effect of .A secondary pathway, which yields HCrO4^-, proceeds by a bimolecular reaction between two CrO2²⁺ ions.It is characterized by a rate constant 6.0 +/- 0.9 M^-1 s^-1 at 25 deg C (ΔH^<*> = 12.0 +/- 1.7 kcal mol^-1, ΔS^<*> = -14 +/- 6 cal mol^-1 K^-1).A structure of the activated complex for this pathway is proposed.The equilibrium constant for homolysis of CrO2²⁺ is K_hom = 1.6 x 10E-12 M and that calculated from the thermodynamic data for heterolysis (CrO2²⁺ <*> Cr³⁺ + O2^-) is K_het = 3 x 10E-8 M.The latter, although kinetically unimportant, is suitable for thermodynamic analysis.The Lewis basicity of O2^- for Cr³⁺ is discussed on the basis of comparisons to cobalt(III)-ammine complexes, other (H2O)5CrX²⁺ complexes, and acid ionization constants.The data do not require a description of CrO2²⁺ as other than a superoxochromium(III) ion.