Covalency in Actinide and Lanthanide Hexachloride Anions

Whether actinide atoms form covalent or ionic bonds is still a matter of debate after many years of study and it remains a challenge for experimentalists and theoreticians. From the experimental side, synchrotron-based ligand K-edge X-ray absorption spectroscopy appears as a promising technique for probing this issue. From the theoretical perspective, quantum chemical simulations should be able to add on a first principle understanding. To tackle this problem, we have applied these techniques on a series of octahedral uranium and lanthanide chloride salts, MCl$_{6}^{n-}$ (M= U, Ce, Pr) in various oxidation states (n=1, 2, 3). We will show hybrid density functional theory calculations that give evidence of the covalent nature of the M-Cl bond in a quantitative manner. This covalency was found to increase for higher oxidation states.