Complexation of chromium(III) by L-alanine: a kinetic study

Abstract

The kinetics of the reaction of substitution of aqua ligands in chromium(III) complexes by Lalanine has been studied with the aid of a spectrophotometric technique in aqueous media under slightly acidic conditions (pH 3.55 – 5.61). The process did not follow the usual pseudofirst order pattern, even under a large excess of amino acid with respect to the metal ion. On the contrary, the rate decreased much faster than a pseudo-first order reaction would. A two consecutive reaction model has been applied involving the formation (rate constant k1) and decay (rate constant k2) of a long-lived intermediate. Both rate constants decreased with the initial concentration of Cr(III) (due to the pH decrease) and increased with the initial concentration of organic ligand (fractional orders), whereas an increase of the medium ionic strength resulted in an increase of k1 and a decrease of k2, both steps presenting base catalysis and the corresponding activation energies being 60.2 ± 3.3 and 83.3 ± 5.9 kJ mol-1. The rate constants for the replacement of an aqua ligand by L-alanine at 25.0 ºC followed the sequence 2+ CrOH (1.77 M-1 s-1) < + 2 Cr(OH) (128 M-1 s-1) < 3 Cr(OH) (3.07  104 M-1 s-1). The UV-Vis spectrum of the long-lived intermediate was intermediary between those of the inorganic reactant and the reaction product. The spectra of the final reacting mixtures revealed the coexistence of at least four different complexes in equilibrium. A mechanism in agreement with the available experimental data has been proposed, involving an elementary reaction sequence for each experimental rate constant, and starting both with the breakage of a Cr(III)-aqua chemical bond as a previous (slow) step to the coordination of the organic ligand