PNNL

Abstract

Chlorine K-edge X-ray absorption spectroscopy (XAS) and ground-state and time-dependent hybrid density functional theory (DFT) were used to probe electronic structure for Oh-MCl6 2-(M = Ti, Zr, Hf, U) and C4v-UOCl5-, and to determine the relative contributions of valence 3d, 4d, 5d, 6d, and 5f orbitals in M-Cl bonding. Spectral interpretations were guided by time-dependent DFT calculated transition energies and oscillator strengths, which agree well with the experimental XAS spectra. The data provide new spectroscopic evidence for the involvement of both 5f and 6d orbitals in actinide-ligand bonding in UCl6 2-. For the MCl6 2-, where transitions into d orbitals of t2g symmetry are spectroscopically resolved for all four complexes, the experimentally determined Cl 3p character per M-Cl bond increases from 8.3(4)% (TiCl 6 2-) to 10.3(5)% (ZrCl6 2-), 12(1)% (HfCl6 2-), and 26 18(1)% (UCl6 2-). Chlorine K-edge XAS spectra of UOCl5- provide additional insights into the transition assignments by 27 lowering the symmetry to C4v, where five pre-edge transitions into both 5f and 6d orbitals are observed. For UCl6 2-, the XAS data 28 suggest that orbital mixing associated with the U 5f orbitals is considerably lower than that of the U 6d orbitals. For both UCl6 2-29 and UOCl5-, the ground-state DFT calculations predict a larger 5f contribution to bonding than is determined experimentally. 30 These findings are discussed in the context of conventional theories of covalent bonding for d- and f-block metal complexes.