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

The coordination chemistry of Cm(III) with aqueous phosphates was investigated by means of laser-induced luminescence spectroscopy and simulations. For the first time, in addition to the presence of Cm(HPO), the formation of Cm(HPO) was unambiguously established from the luminescence spectroscopic data collected at various H concentrations (-log [H] = 2.52, 3.44, and 3.65), ionic strengths (0.5-3.0 mol·L NaClO), 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 and molal entropy Δ of both complexation reactions were derived using the integrated van't Hoff equation and indicated an endothermic and entropy-driven complexation. For the Cm(HPO) complex, a more satisfactory description could be obtained when including the molal heat capacity term. While monodentate binding of the HPO ligand(s) to the central curium ion was found to be the most stable configuration for both complexes in our 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(HPO) 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(HPO) species with increasing temperature.