Kinetics and mechanism of base hydrolysis of chromium(III) complexes with oxalates and quinolinic acid

Base hydrolysis of [Cr(ox)(2)(quin)](3-) (where quin(2-) is N,O-bonded 2,3-pyridinedicarboxylic acid dianion) causes successive ligand dissociation and leads to a formation of a mixture of oligomeric chromium(III) species, known as chromates(III). The reaction proceeds through [Cr(ox)(quin)(OH)(2)](3-) and [Cr(quin)(OH)(4)](3-) formation. Dissociation of oxalato ligands is preceded by the opening of the Cr-quin chelate-ring at the Cr-N bond. The kinetics of the chelate-ring opening and the first oxalate dissociation were studied spectrophotometrically, within the lower energy d-d band region at 0.4-1.0 M NaOH. The pseudo-first-order rate constants (k (obs0) and k (obs1)) were calculated using SPECFIT software for an A -> B -> C reaction pattern. Additionally, kinetics of base hydrolysis of [Cr(ox)(quin)(OH)(2)](3-) and cis-[Cr(ox)(2)(OH)(2)](3-) were studied. The determined pseudo-first-order rate constants were independent of [OH(-)]. A mechanism is postulated that the reactive intermediate with the one-end bonded quin ligand, [Cr(ox)(2)(O-quin)(OH)](4-), formed in the first reaction stage, subsequently undergoes oxalates substitution. Kinetic parameters for the chelate-ring opening and the first oxalate dissociation were determined.