The role of covalency in enhancing stability of Eu and Am complexes: a DFT comparison of BTP and BTPhen

Abstract

We compare the stabilities and bonding nature of [Eu/Am(BTPhen)₂(NO₃)]²⁺ complexes to those previously reported for [Eu/Am(BTP)₃]³⁺, and investigate whether more accurately reflecting the reaction conditions of the separation process by considering [Eu/Am(NO₃)₃(H₂O)_x] (x = 3, 4) complexes instead of aquo complexes increases the selectivity of the separation ligands BTP and BTPhen for Am over Eu. The geometric and electronic structures of [Eu/Am(BTPhen)₂(NO₃)]²⁺ and [Eu/Am(NO₃)₃(H₂O)_x] (x = 3, 4) have been evaluated using density functional theory (DFT) and used as the basis for analysis of the electron density through the application of the quantum theory of atoms in molecules (QTAIM). Increased covalent bond character for the Am complexes of BTPhen over Eu analogues was found, with this increase more pronounced than that found in BTP complexes. BHLYP-derived exchange reaction energies were evaluated using the hydrated nitrates as a reference and a favourability for actinide complexation by both BTP and BTPhen was found, with the BTPhen ligand found to be more selective, with relative stability ≈0.17 eV greater than BTP.