Speciation, thermodynamics and structure of Np(v) oxalate complexes in aqueous solution

Abstract

The speciation, thermodynamics and structure of the Np(V) (as the NpO₂⁺ cation) complexes with oxalate (Ox²⁻) are studied by different spectroscopic techniques. Near infrared absorption spectroscopy (Vis/NIR) is used to investigate complexation reactions as a function of the total ligand concentration ([Ox²⁻]_total), ionic strength (I_m = 0.5–4.0 mol kg⁻¹ Na⁺(Cl⁻/ClO₄⁻)) and temperature (T = 20–85 °C) for determination of the complex stoichiometry and thermodynamic functions (log β⁰_n(T), Δ_rH⁰_n, Δ_rS⁰_n). Besides the solvated NpO₂⁺ ion, two NpO₂⁺ oxalate species (NpO₂(Ox)_n¹⁻²ⁿ; n = 1, 2) are identified. With increasing temperature a decrease of the molar fractions of the 1 : 1 – and 1 : 2 – complexes is observed. Application of the law of mass action yields the temperature dependent conditional stability constants log β′_n(T) at a given ionic strength which are extrapolated to IUPAC reference state conditions (I_m = 0) according to the specific ion interaction theory (SIT). The log β⁰_n(T) values of both complex species (log β⁰₁(25 °C) = 4.53 ± 0.12; log β⁰₂(25 °C) = 6.22 ± 0.24) decrease with increasing temperature confirming an exothermic complexation reaction. The temperature dependence of the thermodynamic stability constants is described by the integrated van't Hoff equation yielding the standard reaction enthalpies (Δ_rH⁰₁ = −1.3 ± 0.7 kJ mol⁻¹; Δ_rH⁰₂ = −8.7 ± 1.4 kJ mol⁻¹) and entropies (Δ_rS⁰₁ = 82 ± 2 J mol⁻¹ K⁻¹; Δ_rS⁰₂ = 90 ± 5 J mol⁻¹ K⁻¹) for the complexation reactions. In addition, the sum of the specific binary ion-ion interaction coefficients Δε⁰_n(T) for the complexation reactions are obtained from SIT modelling as a function of the temperature. The structure of the complexes and the coordination mode of oxalate are investigated using EXAFS spectroscopy and quantum chemical calculations. The results show, that in case of both species NpO₂(Ox)⁻ and NpO₂(Ox)₂³⁻, chelate complexes with 5-membered rings are formed.