Nucleophilic catalysis by HPO42- in the hydrolysis of Cr2O72-. Formation and decay of HO3POCrO32-

Article abstract of DOI:10.1071/CH9942283

The initial reaction of Cr₂O_7(2-) in phosphate buffer (pH 6.03-8.54, 25.0°C, I = 1.0 M NaClO₄) follows the rate law k_obs = kK[HPO_4(2-)]/(1+K[HPO_4(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±1.3 M^-1), and rate-determining loss of CrO_4(2-) from this species (k = 4.4±0.5 s^-1) to give HO₃POCrO_3(2-) (pK_a^c = 6.96). This appears to be the first clear demonstration of an addition-elimination (stepwise) mechanism for substitution at chromium (VI). Subsequent equilibration of HO₃POCrO_3(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 (HPO_4(2-)) catalysis, in addition to a spontaneous reaction.