Covalency in An(Ⅲ/Ⅳ) (An=Pu, Am) Hexachlorides

摘要

Covalency in bonding represents a fundamental conceptfor rationalizing many phenomena in physical, chemical andbiological sciences. Covalency in actinide-ligand bonding isnot only of fundamental interest but also of technologicalimportance in actinide separation in nuclear industry. Howto measure and understand the actinide covalent bondinghas been widely debated and still remain to be addressed.Advanced spectroscopic techniques, such as ligand K-edgeX-ray absorption spectroscopy (XAS), metal M-edgehigh-energy resolution (HR) XAS near-edge structure, 3d4fresonant inelastic X-ray scattering, HR fluorescencedetectionXAS, pulsed electron paramagnetic resonancespectroscopy (EPR), photoluminescence spectrsocpy andnuclear magnetic resonance (NMR), and complexationthermodynamics have been employed together withcomputational analysis to probe the covalent character inactinide-ligand bonds. These studies suggest that theactinide covalency depends on the hard/soft nature ofligand, the oxidation state of An and even energy neardegeneracyof metal and ligand valence orbitals. In thissense, ligand K-edge XAS has advantages in measuring thecovalency not only across actinide series for a specificligand but also in different ligand complexes for specificactinide element. However, most ligand K-edge XASmeasurements have focused on early actinide Th and Ucompounds. This stems from the difficulties incharacterizing the covalent bonding for actinide especiallyfor plutonium and the other transuranium elements due tothe their radioactivity and scarcities.5 In this work, weinvestigated actinide-ligand orbital mixing in PuCl_6~(2−),PuCl_6~(3−) and AmCl_6~(3−) complexes by Cl K-edge XASmeasurement and the relativistic density functional tounderstand the An-Cl bond covalency as a function of Anoxidation state and element.