Mixed Ligand Complexes of Am3+, Cm3+ and Eu3+ with HEDTA and HEDTA plus NTA-Complexation Thermodynamics and Structural Aspects

The stability constants and the associated thermodynamic parameters of formation for the 1:1 binary complexes of Am3+, Cm3+ and Eu3+ with N-(2-hydroxyethyl) ethylenediaminetriacetate (HEDTA) and their 1:1:1 ternary complexes with HEDTA + NTA (nitrilotriacate) were determined by distribution ratio measurements using solvent extraction in aqueous solutions of =0.10 mola <...L-1 (NaClO4) at temperatures of 0-45 A degrees C. Formation of these complexes is favored by both the enthalpy (exothermic) and the entropy (endothermic) terms. Luminescence lifetime measurements with Cm and Eu were used to study the coordination environment of these complexes over a range of concentrations and pH values. In the binary complexes M(HEDTA), HEDTA is a hexadentate ligand with three waters of hydration, while in the ternary complexes M(HEDTA)(NTA)(3-) we propose that the HEDTA retaines hexadentate coordination with NTA binding via three sites, depending on the pH of the solution, with the observation that the complex may contain a single water of hydration.