NUCLEOPHILIC CATALYSIS BY HPO42- IN THE HYDROLYSIS OF CR2O72- - FORMATION AND DECAY OF HO3POCRO32-

The initial reaction of Cr2O72- in phosphate buffer (pH 6.03-8.54, 25.0 degrees C, I = 1.0 M NaClO4) follows the rate law k(obs) = kK[HPO42-]/(1+K[HOP42-]). This is interpreted as arising from the reversible and rapid formation of a chromium(vr)-phosphato intermediate of increased coordination number (K = 5.5+/-1.3 M(-1)), and rate-determining loss of CrO42- from this species (k = 4.4+/-0.5 s(-1)) to give HO3POCrO32- (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 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.