Volume 34, Number 1, 75-84, DOI: 10.1007/s11243-008-9160-1

Chromium(III) complexes with lutidinic acid: kinetic studies in HClO4 and NaOH solutions

Ewa Kita, Hasan Marai and Łukasz Iglewski

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Abstract

Chromium(III)-lutidinato complexes of general formula [Cr(lutH) n (H2O)6−2n ]3−n (where lutH is N,O-bonded lutidinic acid anion) were obtained and characterized in solution. Acid-catalysed aquation of [Cr(lutH)3]0 leads to only one ligand dissociation, whereas base hydrolysis produces chromates(III) as a result of subsequent ligand liberation steps. The kinetics of the first ligand dissociation were studied spectrophotometrically, within the 0.1–1.0 M HClO4 and 0.4–1.0 M NaOH range. In acidic media, two reaction stages, the chelate-ring opening and the ligand dissociation, were characterized. The dependencies of pseudo-first-order rate constants on [H+] are as follows: k obs1 = k 1 + k −1/K 1[H+] and k obs2 = k 2 K 2[H+]/(1 + K 2[H+]), where k 1 and k 2 are the rate constants for the chelate-ring opening and the ligand dissociation, respectively, k −1 is the rate constant for the chelate-ring closure, and K 1 and K 2 are the protonation constants of the pyridine nitrogen atom and coordinated 2-carboxylate group in the one-end bonded intermediate, respectively. In alkaline media, the rate constant for the first ligand dissociation depends on [OH]: k obs1 = k OH(1) + k O[OH], where k OH(1) and k O are rate constants of the first ligand liberation from the hydroxo- and oxo-forms of the intermediate, respectively, and K 2 is an equilibrium constant between these two protolytic forms. Kinetic parameters were determined and a mechanism for the first ligand dissociation is proposed. The kinetics of the ligand liberation from [Cr(lut)(OH)4]3− were also studied and the values of the pseudo-first-order rate constants are [OH] independent.

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