Formation of actinide(IV) carboxylate complexes in aqueous solution – the unexpected predominance of hexanuclear species

Tetravalent actinides form strong complexes with carboxyl containing ligands. Such complexes play an important role in technological processes as well as biological and environmental systems. The aqueous chemistry of such complexes is therefore of wide interest. So far, most of the thermodynamic data of actinide(IV) carboxylates are estimated assuming mononuclear solution species (see for example [1]). We applied comprehensive spectroscopic studies (EXAFS, UV-Vis-NIR spectroscopy and X-ray diffraction) to estimate the complex structure of actinide(IV) carboxylates with different actinides (Th4+, U4+ and Np4+) and carboxylates (RCOO–; R = H, CH3, CHR’NH2; R’ = H, CH3, CH2SH) in aqueous solution and solid state [2-4]. These studies show clearly that in all of the investigated systems hexanuclear complexes appear, which become predominant with increasing metal and ligand concentration as well as increasing pH, and comprise finally often close to 100% of the species distribution. The aim of our studies was to investigate the complex structure, stability constants, and the mechanism of complex formation.