A kinetic study on the iron(III)-promoted aquation of [Cr(C2O4)2(picolinato)]2- and [Cr(C2O4)2(2-pyridineacetato)]2- complexes

Two [Cr(C2O4)(2)(AB)](2-) type complexes, obtained from the reaction of cis-[Cr(C2O4)(2)(H2O)(2)](-) with the AB ligand, [AB = picolinic (pyac) or 2-pyridine-ethanoic acid (pyeac) anions], were converted into [Cr(C2O4)(pyac)(H2O)(2)](0) and [Cr(C2O4)(pyeac)(H2O)(2)](0) compounds, respectively via Fe-III-induced substitution of the oxalato ligand. The aquation products were separated chromatographically and their spectral characteristics and acid dissociation constants determined. The kinetics of the oxalato ligand substitution were studied with a 10-40 fold excess of Fe-III over [Cr-III] at [H+] = 0.2 M and at constant ionic strength 1.0 M (Na+, H+, Fe3+, ClO4-). The reaction rate law is of the form: r = k(obs)[Cr-III], where k(obs) = kQ[Fe-III]/(1 + Q[Fe-III]). The first-order rate constants (k), preequilibria quotients (Q) and activation parameters derived from the k values have been determined. The reaction mechanism is discussed in terms of a Lewis acid catalyzed (induced) ligand substitution.