SYNTHESIS, ACID-BASE PROPERTIES AND KINETICS OF H-AQ+-CATALYZED AQUATION OF [CR(C2O4)2(PYRIDOXIC ACID)]2- ION

The [Cr(C2O4)2(HPA)]2- complex (PA=4-pyridoxic acid*) was isolated chromatographically as a by-product of the reaction between pyridoxal and cis-[Cr(C2O4)2(OH2)2]- ion. The dependence of its electronic spectrum on pH is consistent with a bidentate mode of coordination for PA through the phenolic and the carboxylate oxygen atoms. The pK(a) value for deprotonation of the pyridinium nitrogen is 6.6 (298 K, l-0.1). A slow substitution of the PA ligand by water molecule is observed in acidic media. The reaction is strongly catalyzed by H(aq)+ ions; the rate law is: k(obs) = k1[H+] + k2[H+]2. The activation parameters for both reaction paths have been determined.