Covalency in Ln-Cl bonds of Oh-LnCl6(x-) (x = 3 for Ln = Ce(III), Nd(III), Sm(III), Eu(III), Gd(III); x = 2 for Ln = Ce(IV)) anions has been investigated, primarily using Cl K-edge X-ray absorption spectroscopy (XAS) and time-dependent density functional theory (TDDFT); however, Ce L3,2-edge and M5,4-edge XAS were also used to characterize CeCl6(x-) (x = 2, 3). The M5,4-edge XAS spectra were modeled using configuration interaction calculations. The results were evaluated as a function of (1) the lanthanide (Ln) metal identity, which was varied across the series from Ce to Gd, and (2) the Ln oxidation state (when practical, i.e., formally Ce(III) and Ce(IV)). Pronounced mixing between the Cl 3p- and Ln 5d-orbitals (t2g* and eg*) was observed. Experimental results indicated that Ln 5d-orbital mixing decreased when moving across the lanthanide series. In contrast, oxidizing Ce(III) to Ce(IV) had little effect on Cl 3p and Ce 5d-orbital mixing. For LnCl6(3-) (formally Ln(III)), the 4f-orbitals participated only marginally in covalent bonding, which was consistent with historical descriptions. Surprisingly, there was a marked increase in Cl 3p- and Ce(IV) 4f-orbital mixing (t1u* + t2u*) in CeCl6(2-). This unexpected 4f- and 5d-orbital participation in covalent bonding is presented in the context of recent studies on both tetravalent transition metal and actinide hexahalides, MCl6(2-) (M = Ti, Zr, Hf, U).

Chlorine K-edge X-ray absorption spectroscopy (XAS) and ground-state and time-dependent hybrid density functional theory (DFT) were used to probe the electronic structures of O(h)-MCl(6)(2-) (M = Ti, Zr, Hf, U) and C(4v)-UOCl(5)(-), 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 UCl(6)(2-). For the MCl(6)(2-), where transitions into d orbitals of t(2g) 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)% (ZrCl(6)(2-)), 12(1)% (HfCl(6)(2-)), and 18(1)% (UCl(6)(2-)). Chlorine K-edge XAS spectra of UOCl(5)(-) provide additional insights into the transition assignments by lowering the symmetry to C(4v), where five pre-edge transitions into both 5f and 6d orbitals are observed. For UCl(6)(2-), the XAS data suggest that orbital mixing associated with the U 5f orbitals is considerably lower than that of the U 6d orbitals. For both UCl(6)(2-) and UOCl(5)(-), the ground-state DFT calculations predict a larger 5f contribution to bonding than is determined experimentally. These findings are discussed in the context of conventional theories of covalent bonding for d- and f-block metal complexes.

Evidence for metal-carbon orbital mixing in thorocene and uranocene was determined from DFT calculations and carbon K-edge X-ray absorption spectra (XAS) collected with a scanning transmission X-ray microscope (STXM). Both the experimental and computational results showed that the 5f orbitals engaged in significant d-type mixing with the C 8 H 8 2À ligands, which increased as the 5f orbitals dropped in energy on moving from Th 4+ to U 4+. The first experimental evidence for extensive f-orbital interactions has been provided by the C K-edge XAS analysis of thorocene; however, f-type covalency in uranocene was negligible. The results highlighted two contrasting trends in orbital mixing from one pair of highly symmetric molecules, and showed that covalency does not increase uniformly for different molecular orbital interactions with later actinides.

Abstract:We describe the use of CI K-edge X-ray Absorption Spectroscopy (XAS) and both ground state and time-dependent hybrid-Density Functional Theory (OFT) to probe electronic structure and determine the degree of orbital mixing in M-CI bonds for (C5Me5hMCI2 (M =Ti, 1; Zr, calculations as a guide to interpret the experimental CI K-edge XAS, these experiments suggest that when evaluating An-CI bonding, both 5f-and 6d-orbitals should be considered. For (C5Me5)zThCb, the calculations and XAS indicate that the 5f-and 6d-orbitals are nearly degenerate and heavily mixed. In contrast, the 5f-and 6d-orbitals in (C5Mes)2UCl z are no longer degenerate, and fall in two distinct energy groupings. The Sf-orbitals are lowest in energy and split into a 5-over-2 pattern with the high lying U 6d-orbitals split in a 4-over-1 pattern,the latter of which is similar to the d-orbital splitting in group IV transition metal (C 5 Rs)zMCl z (R =H, Me)compounds. Time dependent-OFT (TO-OFT) was used to calculate the energies and intensities of CI is transitions into empty metal based orbitals containing CI 3p character, and provide simulated CI K-edge XAS spectra for 1-4. However, for 5, which has two unpaired electrons, analogous information was obtained from transition dipole calculations. The simulations provide additional confidence in the interpretation of spectra based on ground state calculations.Overall, this study demonstrates that CI K-edge XAS and DFT calculations represent powerful J20ls that can be used to experimentally evaluate electronic structure and covalency in actinide metal-ligand bonding. In addition, fhese results provide a framework that can be used in future studies to evaluate actinide covalency in compounds that contain transuranic elements.