Thermodynamics of dioxoneptunium(V) complexation by dicarboxylic acids

Abstract

The free energy and enthalpy changes associated with complexation of NpO2+by oxydiacetic acid, iminodiacetic acid, and malonic acid have been measured in 1 M NaClO4at 25 °C using spectrophotometric, potentiometric, and calorimetric titrations. Only 1:1 complexes were observed with the oxydiacetate and iminodiacetate dianions, while both 1:1 and 1:2 complexes were observed with the malonate dianion. The free energies indicate that all of the potential donor groups are bound to the neptunium center. The favorable complexation entropies are unusually large for the NpO2+-oxydiacetate complex, suggesting a unique interaction of the metal-ligand complex with the solvent molecules. Relationships between the free energies of complexation of UO22+and NpO2+and between the dicarboxylate complexation entropies of NpO2+and a variety of metal cations are developed and discussed.