Thermodynamics of the complex formation between thorium(IV) and some polydentate ligands in aqueous solution

Abstract

The changes in free energy, enthalpy, and entropy for the formation of thorium(IV)–oxydiacetate, –iminodiacetate, –thiodiacetate, and –succinate complexes have been determined by potentiometric and calorimetric titrations at 25 °C in aqueous 1 mol dm^–3 sodium perchlorate. All the ligands form 1:1 chelate complexes with the thorium(IV) ion the stability of which is dependent on both the chelate ring dimensions and the nature of the donor group in the chain. The order of the relative stabilities (iminodiacetate > oxydiacetate > thiodiacetate > succinate) is mainly dependent on the reaction enthalpies, since the ΔS^⊖ values are close to each other. In the thorium(IV)–oxydiacetate system the maximum number of three ligands for every metal ion was reached. Because of precipitation of solid compounds in the other systems, it was only possible to define complexes with a lower number of co-ordinated ligands: two for succinate and thiodiacetate, and one for iminodiacetate. Owing to the lower stability of the chelate ring of thiodiacetate and succinate complexes and the high basicity of the amino-group of iminodiacetate, these ligands form also unchelated protonated complexes.