Complexation of thorium(IV) with 2-furoic acid and 2-thenoic acid in aqueous solution

Complexation of Th(IV) with 2-furoic acid (C4H3OCOOH) and 2-thenoic acid (C4H3SCOOH) in 1.0 mol dm−3 NaClO4 has been studied at 25 °C. The formation constants of successive complexes ThLj(4−j)+, where L stands for the furoate or thenoate anion and j = 1–3 for 2-furoic acid and j = 1–2 for 2-thenoic acid, and corresponding molar enthalpies of complexation were determined by potentiometry and calorimetry. Spectroscopic techniques, including FT Raman and Extended X-Ray Absorption Fine Structure (EXAFS) were used to acquire further information on the interaction between Th(IV) and the ligands. Both the thermodynamic and spectroscopic data are consistent with the formation of ThLj(4−j)+ complexes, in contrast to earlier results in the literature which suggest that the protonated complexes ThL4H22+ and ThL3H23+ are dominant under the experimental conditions. The binding strength of furoic acid and thenoic acid with Th(IV) is discussed in conjunction with the data for other monocarboxylic acids.