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(H₂PO₄)²⁺, the formation of Cm(H₂PO₄)₂⁺ was unambiguously established from the luminescence spectroscopic data collected at various H⁺ concentrations (-log₁₀ [H⁺] = 2.52, 3.44, and 3.65), ionic strengths (0.5-3.0 mol·L^-1 NaClO₄), 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_RH_m⁰ and molal entropy Δ_RS_m⁰ of both complexation reactions were derived using the integrated van't Hoff equation and indicated an endothermic and entropy-driven complexation. For the Cm(H₂PO₄)₂⁺ complex, a more satisfactory description could be obtained when including the molal heat capacity term. While monodentate binding of the H₂PO₄^- 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(H₂PO₄)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(H₂PO₄)₂⁺ species with increasing temperature.