Complexation of thorium(IV) with malonate at variable temperatures

Complexation between thorium(IV) and malonate in 1.05 mol kg(-1) NaClO4 was studied from 10 to 70 degrees C by potentiometry and calorimetry. Three complexes, ThL2+, ThL2 and ThL32-, where L stands for malonate, were identified. The stability constants, the enthalpies and entropies of complexation at variable temperatures were determined. Despite that the enthalpies of complexation become more endothermic, the complexes become more stable due to increasingly larger entropies of complexation that exceed the unfavorable contribution of enthalpies of complexation at higher temperatures. The thermodynamic trends are explained in terms of an electrostatic model and the change in the structure of water with temperature. The data are compared with previous results on the complexation of U(VI) and Nd(III) with malonate and other carboxylate ligands.