Thermodynamic Considerations of Covalency in Trivalent Actinide-(poly)aminopolycarboxylate Interactions

The complexation thermodynamics of trivalent actinides with (poly)aminopolycarboxylates (APCs) are reviewed to assess aspects of covalency and selectivity in actinide-amine interactions. The preferential interaction of APC ligands with trivalent actinides over trivalent actinides has been interpreted to suggest that the amine donors on the APC ligand are able to interact covalently with the actinides. This potentially covalent interaction could allow APC ligands to serve as a thermodynamic probe for covalency in actinide interactions. This review considers enthalpic binding signatures associated with actinide-APC systems, linear free energy relationships that compare the chemistry of comparably sized trivalent lanthanides and actinides and, through examination of the TALSPEAK system, evaluation of a separation system where understanding the chemistry of the heaviest actinides could be relevant. An overarching observation of this review is the lack of thermodynamic data that would be instructive in describing the chemistry of a broader part of the actinide series.