Ligand Sequential Replacement on Chromium(III)–Aqua Complexes by l‑Alanine and Other Biological Amino Acids: A Kinetic Perspective

The ligand sequential replacement on chromium­(III)–aqua complexes by l-alanine in slightly acidic aqueous solutions (pH range: 3.55–5.61) has been kinetically followed by means of UV–vis spectrophotometry. A two rate constant model has been applied to fit the absorbance–time data, corresponding to the formation (k1) and decay (k2) of an intermediate not reactive enough to be in steady state (long-lived intermediate). The kinetic orders of the amino acid were fractional (0.40 ± 0.03 for k1 and 0.40 ± 0.02 for k2). The two steps showed base catalysis, and the activation energies were 60 ± 3 (for k1) and 83 ± 6 (for k2) kJ mol–1. The rate constants for the coordination of the first l-alanine ligand followed the sequence CrOH2+ 2+ 3, Cr3+ being almost inactive. This suggests that the increase in the reaction rate with increasing pH was caused by the enhancement of the lability of the Cr­(III)–aqua bonds induced by the presence of hydroxo ligands. The activation parameters for a series of ligand substitution on Cr­(III)–aqua complexes by organic molecules yielded a statistically significant enthalpy–entropy linear plot with an isokinetic temperature of 296 ± 21 K