THERMODYNAMIC PARAMETERS OF COMPLEXATION OF LANTHANOIDS(III) BY ALPHA-SUBSTITUTED CARBOXYLATE ANIONS

The protonation enthalpy of alpha-hydroxyphenylacetate, alpha-phenylla ctate, and alpha-methoxyphenylacetate anions and the complexation enth alpy of lanthanoid(III) with these carboxylate ions have been measured by calorimetric technique in 0.1 mol/dm3 (NaClO4) at 25-degrees-C. Th e results are compared to those of the phenylacetate complexation. The low protonation enthalpy of alpha-hydroxycarboxylate relative to othe r monocarboxylates is attributed to intra hydrogen-bond formation. The difference between DELTAH101 of alpha-hydroxycarboxylates of light la nthanoid(III) and those of the heavy ions is smaller than the correspo nding difference for complexation by phenylacetate. By contrast, the d ifference in DELTAS101 for complexation of light and heavy ions by the two ligands is similar. Complexation by alpha-methoxyphenylacetate, h owever did not follow either pattern.