Nucleophilic Catalysis by HPO42- in the Hydrolysis of Cr2O72-. Formation and Decay of HO3POCrO32-

Abstract

The initial reaction of Cr2O72- in phosphate buffer (pH 6.03-8.54, 25.0°C, I = 1.0 M NaClO4) follows the rate law kobs = kK [HPO42]/(1+K[HPO42-]). 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 CrO42- from this species (k = 4.4�0.5 s-1) to give HO3POCrO32- ( pKc 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 HO3POCrO32- 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 (HPO42-) catalysis, in addition to a spontaneous reaction.