Nucleophilic Catalysis by HPO₄^2- in the Hydrolysis of Cr₂O₇^2-. Formation and Decay of HO₃POCrO₃^2-

Abstract

The initial reaction of Cr₂O₇^2- in phosphate buffer (pH 6.03-8.54, 25.0°C, I = 1.0 M NaClO₄) follows the rate law k_obs = kK [HPO₄²]/(1+K[HPO₄^2-]). This is interpreted as arising from the reversible and rapid formation of a chromium(VI)- phosphato intermediate of increased coordination number (K = 5.5±1.3M^-1), and rate-determining loss of CrO₄^2- from this species (k = 4.4±0.5 s^-1) to give HO₃POCrO₃^2- ( pK^c _a= 6.96). This appears to be the first clear demonstration of an addition-elimination (stepwise) mechanism for substitution at chromium (VI). Subsequent equilibration of HO₃POCrO₃^2- to give HCrO₄- and H₂PO₄- (K = 5.95±1.90 M^-1) is seen as a separate process which is subject to specific H⁺ and OH^-, and general base (HPO₄^2-) catalysis, in addition to a spontaneous reaction.