Chromium(III)-lutidinato complexes of general formula [Cr(lutH) n (H 2 O) 6-2n ] 3-n (where lutH -is N,Obonded 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 HClO 4 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 oxoforms 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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