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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 ( k 1 ) and decay ( k 2 ) 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 k 1 and 0.40 ± 0.02 for k 2 ). The two steps showed base catalysis, and the activation energies were 60 ± 3 (for k 1 ) and 83 ± 6 (for k 2 ) kJ mol -1 . The rate constants for the coordination of the first l-alanine ligand followed the sequence CrOH 2+ &lt; Cr(OH) 2  + &lt; Cr(OH) 3 , Cr 3+ 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.

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