Revisiting the Complexation of Cm(III) with Aqueous Phosphates: What Can We Learn from the Complex Structures Using Luminescence Spectroscopy and Ab Initio Simulations?

The coordination chemistry of Cm(III) with aqueous phosphates was investigated by means of laser-induced luminescence spectroscopy and ab initio simulations. For the first time, in addition to the presence of Cm(H 2 PO 4 ) 2+, the formation of Cm(H 2 PO 4 ) 2 + was unambiguously established from the luminescence spectroscopic data collected at various H + concentrations (−log 10 [H + ] = 2.52, 3.44, and 3.65), ionic strengths (0.5–3.0 mol·L –1 NaClO 4 ), and temperatures (25–90 °C). Complexation constants for both species were derived and extrapolated to standard conditions using the specific ion interaction theory. The molal enthalpy Δ R H m 0 and molal entropy Δ R S m 0 of both complexation reactions were derived using the integrated van’t Hoff equation and indicated an endothermic and entropy-driven complexation. For the Cm(H 2 PO 4 ) 2 + complex, a more satisfactory description could be obtained when including the molal heat capacity term. While monodentate binding of the H 2 PO 4 – ligand(s) to the central curium ion was found to be the most stable configuration for both complexes in our ab initio simulations and luminescence lifetime analyses, a different temperature-dependent coordination to hydration water molecules could be deduced from the electronic structure of the Cm(III)–phosphate complexes. More precisely, where the Cm(H 2 PO 4 ) 2+ complex could be shown to retain an overall coordination number of 9 over the entire investigated temperature range, a coordination change from 9 to 8 was established for the Cm(H 2 PO 4 ) 2 + species with increasing temperature.