Thermodynamics on Complexation of Uranium(VI) with Nitrilotri-3-Propanoic Acid and Tris(2-Carboxyethyl) Phosphine in Aqueous Solution

Uranyl, a strong Lewis acid, forms stable complexes with relatively hard donors and it is commonly believed that uranyl complex with trialkylamine is stabler than trialkylphosphine. However, there are no thermodynamic parameters to evaluate the complexation stabilities of R 3 N-UO 2 2+ and R 3 P-UO 2 2+ . Nitrilotri-3-propanoic Acid (NTPA) and Tris(2-carboxyethyl) phosphine (TCEP) possess similar skeleton. The set of isostructural ligands is a perfect pair to assess the differences of uranyl (UO 2 2+ ) complexes with trialkylamine versus trialkylphosphine. The protonation reactions of NTPA and TCEP were studied by potentiometry and calorimetry, and the uranyl complexation with both the ligands were studied by potentiometry. The results indicate that central trialkylamine in NTPA has a higher affinity for the proton than trialkylphosphine in TCEP. The complexation of uranyl with NTPA is stabler than TCEP with 1.2 logarithmic units. Striking differences imply the central nitrogen of NTPA and phosphorus of TCEP definitely participate in uranyl complexation. The higher stability of UO 2 2+ /NTPA versus UO 2 2+ /TCEP is intuitively attributed to stronger bonding of N-UO 2 2+ than P-UO 2 2+ because the only differences between TCEP and NTPA are the central atoms.