Lanthanide croconate complexation in solution

The thermodynamic parameters of complexation of lanthanide cations by croconate ligands have been measured in aqueous solution using potentiometry and calorimetry. Comparison of experimental and calculated (with a modified Born equation) values of ..delta..G indicates the presence of aromatic pi systems in the carbon rings of the ligands in squarate, croconate and tropolonate complexes but of a nonaromatic pi system in the cyclic ether rings of kojate and maltolate complexes.