Thermodynamics of complexation of lanthanides by benzoic and isophthalic acids

The thermodynamic parameters (..delta..G, ..delta..H, and ..delta..S) of formation of the 1:1 complexes between lanthanide cations and benzoate and isophthalate anions were determined by potentiometric and calorimetric titrations in aqueous solutions of 0.10 M (NaClO/sub 4/) ionic strength. The values for the benzoate complexes agreed with the general relationships for alkyl monocarboxylate complexation (e.g., log ..beta../sub 101/ vs. pK/sub a/). Even though the isophthalate complexes must also involve interaction between the metal and a single carboxylate group, the thermodynamic parameters differ from those for benzoate complexing. It is proposed that the enhanced stability of the isophthalate complexes is related to polarization of electronic charge from the unbound carboxylate group through the ..pi.. system of the phenyl ring to the bonding carboxylate.