Illuminating the nature of f-element bonding with ‘soft’ ligands
Preparation of sulfur and selenium donor complexes advances actinide science and separations

An intimate knowledge of the fundamental coordination chemistry of the actinides is vital to understanding processes and separations for the minimization and treatment of nuclear waste. One possible strategy to reduce the volume and storage time of high-level waste is partitioning and transmutation. In this approach the long-lived minor actinide isotopes are placed in a reactor and subjected to a neutron flux causing them to be transmuted via nuclear reactions to isotopes with shorter half-lives, which decreases storage costs.

Before transmutation, it is crucial to separate the trivalent actinides, referred to here as An(III), from trivalent lanthanides, Ln(III), because some of the lanthanides are “neutron poisons” and the amount of material put through the reactor should be minimized. The separation of An(III) and Ln(III) is particularly difficult because the generally ionic nature of the bonding for “hard” metal ions means many extractants display no selectivity between An(III) and Ln(III) of similar ionic radii, such as americium(III) and europium(III).

However, the sulfur-bearing Cyanex extractants (dithiophosphinic acids, which have a negative charge delocalized over two sulfur atoms) display an exceptional preference for An(III) over Ln(III). Conclusive experimental evidence for the origin of this preference is lacking. The prevailing theory is that because sulfur is a “softer” (less electronegative), more polarizable donor atom, subtle differences in covalent contributions to the bonding may provide selectivity.

The rationale for this hypothesis is that relativistic effects are more pronounced for the actinides than for the lanthanides. Consequently, the 6d and 5f orbitals of An(III) are more spatially diffuse than the 5d and 4f orbitals of Ln(III) of similar ionic radii. Therefore, there exists potential for greater overlap (covalency) between the metal and ligand orbitals for the actinides than for the lanthanides. The difference in energies of the valence orbitals between the trivalent actinides and lanthanides in relation to the valence ligand orbitals also contributes to actinide-ligand interactions.

Despite the importance of actinide and lanthanide separations, relatively few research efforts have focused on understanding the extraction behavior of Cyanex reagents from a coordination chemistry perspective; one of our key objectives is to help fill this gap in knowledge. Historically, studies of covalency in actinide bonding use complexes in which most of the coordination sites are occupied by sterically demanding functionalities to focus covalency toward one or two metal-ligand bonds.

Our research has a slightly more applied flavor in the sense that we are synthesizing homoleptic compounds (i.e., the entire actinide coordination sphere is available for bonding by the same ligand) with nitrogen (N), sulfur (S), and selenium (Se) donor atoms, providing coordination environments relevant to An(III) and Ln(III) separations. By taking a systematic approach to synthesizing actinide complexes with ligands of differing electronegativities, or softnesses, and comparing these complexes with related lanthanide complexes, we aim to determine a structural and coordination basis for preferences in such complexes and assess subtle differences in the bonding between the 5f and 4f elements.

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