Thermodynamics and Structure of Neptunium(V) Complexes with Formate. Spectroscopic and Theoretical Study

The temperature and ionic strength dependences of the complex formation of NpO₂⁺ with formate in aqueous solution are studied by absorption spectroscopy (I_m = 0.5-4.0 mol kg^-1, T = 20-85 °C, [Form^-]_total = 0-0.65 mol kg^-1), extended X-ray absorption fine structure spectroscopy (EXAFS) and quantum chemical methods. The complex stoichiometry and the thermodynamic functions of the complexation reactions are determined by peak deconvolution of the absorption spectra and slope analyses. Besides the solvated NpO₂⁺ ion, two NpO₂⁺ formate species (NpO₂(Form)^1-; n = 1, 2) are identified. Application of the law of mass action yields the temperature dependent conditional stability constants log β'(T) at a given ionic strength. These data are extrapolated to IUPAC reference state conditions (I_m = 0) using the specific ion interaction theory (SIT). The results show, that log β⁰₁(20 °C) = 0.67 ± 0.04 decreases by approximately 0.1 logarithmic units with increasing temperature, log β⁰₂(20 °C) = 0.11 ± 0.11 increases by about 0.2 logarithmic units. The temperature dependence of the log β⁰(T) values is modeled with the integrated Van't Hoff equation yielding the standard reaction enthalpy Δ_rH⁰ and entropy Δ_rS⁰ of the complexation reactions. The results show that the formation of NpO₂(Form) is exothermic (Δ_rH⁰₁ = -2.8 ± 0.9 kJ mol^-1) whereas the formation of NpO₂(Form)₂^- is endothermic (Δ_rH⁰₂ = 6.7 ± 4.1 kJ mol^-1). Furthermore, the binary ion-ion interaction coefficients ε_T(i,k) of the formed complexes are determined in NaClO₄ and NaCl media as a function of the temperature. The coordination mode of formate toward the metal ion is investigated by EXAFS spectroscopy and quantum chemical calculations. A coordination of the ligand via only one O atom of formate to the metal ion is identified.