Actinide speciation using synchrotron radiation

The actinides (An) correspond to the elements in line 7 of the periodic table [AF 3 490] . Their valence orbitals are 5f and 6d, and they have many degrees of oxidation (from III to VII), giving them a rich and complex chemistry in both liquid and solid phases [BN 3 520] . Some actinides are also unusual in that they do not exist as single ions ${\mathit{An}}^{p+}$ but associate with oxygen to form ${\mathrm{AnO}}_2^{2+}$ . This is the case with uranium, neptunium and plutonium in degree VI, which form actinyl ions. Actinide ions, like many ions in chemistry, can be found in different symmetries for a given degree of oxidation. This opens up a wide range of coordination possibilities, both in solution and in the solid state.

Speciation, i.e. knowledge of an element's oxidation state and the nature of its coordination polyhedron, is therefore one of the fundamental data required by physical chemists studying the properties of these radionuclides in the condensed phase (solid compounds, solutions, molten media). In the molecular chemistry of actinide cations, or in the solid-state chemistry of actinide-containing compounds, understanding bonding modes is therefore fundamental to understanding the complex properties of 5f orbitals. In addition to structural data - the basic starting point for a better understanding of bonding modes - electronic properties, particularly those of valence states, are essential. However, although the nature of the bond is a necessary parameter for understanding the behavior of these elements, it is difficult to access experimentally and, by its very nature, difficult to quantify. In fact, it depends on several interdependent parameters, such as the extension of the orbitals involved in the bond (ionocovalence), the nature of the ligands, the valence state of the actinide and the symmetry of the coordination polyhedron. Knowledge of the electronic structure of the metal ion in relation to its environment is therefore essential.