Describing Covalency in Actinide Bonds

Out of interest in better exploring the nature of actinide bonding, investigations have been undertaken which confront the problem of the unusual character of covalency as it occurs in the 5f elements by utilizing multiple analytical frameworks. In this way, nuances of the bonding character which might be invisible to any one theory alone can be illuminated. A series of complexes of lanthanides and actinides with the ligand bis(O, O’-diethyl)dithiophosphate have been synthesized and characterized by optical spectroscopy, NMR spectroscopy, single crystal X-ray diffraction, CASSCF and DFT simulation, and bonding analysis via the Quantum Theory of Atoms in Molecules, the Interacting Quantum Atom, and Ligand Field DFT. These techniques combined expose in addition to the expected enhancement of covalent character in the actinides over the lanthanides, a variation in the source of that covalency between the adjacent actinides plutonium and americium. We are also able to identify a variable which has previously remained unremarked as impacting the covalency of bonds, that being the anisotropy of the metal ions. These results are used as footing to recommend future routes of inquiry with the potential to open new routes to solving globally important problems of f-element separation. Additionally, recent research has suggested anomalous bonding behavior in the complex of californium with a well-known chelating ligand. To investigate this, a series of complexes of lanthanides and californium with the ligand diethylenetriaminepentaacetate, better known as DTPA, have been synthesized and characterized by solution and solid state optical spectroscopy and single crystal X-ray diffraction. Our results describe the very similar stability of multiple bonding archetypes, the high sensitivity of M-O bond lengths to the environment, and illustrate the unusual chemistry of californium as it interacts with this ligand.