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.

Covalent An-Cl bonding in series of +4 actinide hexachlorides, AnCl6 2-(An IV = U, Th, Np, Pu) have been characterized using Cl K-edge XAS and DFT. The results suggest that the 6d-orbital mixing is more substantial than that of the 5f-orbital. Additionally, the results indicate that 5fcovalent bonding with the Cl 3p orbitals is more substantial for Pu than for Th, U, and Np.

X-ray absorption spectroscopy using fluorescence and transmission (via a scanning transmission X-ray microscope), and linear-response density functional theory. The results suggest that moving from Group 6 to Group 7 or down the triads increases M-O e* (π*) mixing. Meanwhile, t 2 * mixing (σ* + π*) remains relatively constant within the same Group. These unexpected changes in frontier orbital energy and composition are evaluated in terms of periodic trends in d orbital energy and radial extension. 3Introduction.The nature of chemical bonds between metals and light atoms such as oxygen, nitrogen, and carbon is of widespread interest because these interactions control the physics and chemistry of many technologically important processes and compounds. Light atoms are prone to form highly covalent metalligand multiple bonds involving one σ bond and one or more π bonds, resulting in oxo, imido, nitrido, alkylidene, alkylidyne, and carbido functionalities with many desirable chemical reactivities and physical properties. 1 Among this diverse group, metal oxides stand out because of their widespread presence in biological and bio-inspired processes and for applications utilizing their unique magnetic, electronic, and thermal properties. [2][3][4][5][6][7][8][9][10][11][12][13] Developing a clear understanding of how M-O electronic structure and orbital mixing changes for a range of metal oxo compounds and materials will greatly benefit attempts to advance these technologies.Among approaches explored previously, ligand K-edge X-ray absorption spectroscopy (XAS) has emerged as an effective method for quantitatively probing electronic structure and orbital mixing in metal-chlorine and metal-sulfur bonds. 14 This spectroscopic technique probes bound state transitions between core ligand 1s orbitals and unoccupied molecular orbitals. The excitations can only have transition intensity if the empty acceptor orbitals contain ligand p character. 14 At first glance, such an approach seems well-suited for studying metal-oxygen bonding. However, attempts to use this technique to study non-conducting molecular systems are complicated by experimental barriers derived from the low energy of the oxygen K-edge (ca. 530 eV), which magnifies issues associated with surface contamination, saturation, and self-absorption effects.In this manuscript, we overcome these challenges and evaluate relative changes in metal-oxo electronic structure and orbital mixing for non-conducting molecular solids using O K-edge spectroscopy in conjunction with hybrid density functional theory (DFT) calculations. Specifically, non-resonant inelastic X-ray scattering (NIXS), XAS, and linear-response density functional theory (TDDFT) are used as Density functional theory calculations using relativistic effective core potentials (RECPs) were conducted to determine how antibonding molecular orbital compositions and energies varied as (1) metals 5 changed within a group (Cr, Mo, W and Mn, Tc, Re) and (2) metal charges increased from M 6+ (Group 6) to M 7+ (Group 7)...

For 3-5d transition-metal ions, the (C5R5)2MCl2 (R = H, Me for M = Ti, Zr, Hf) bent metallocenes represent a series of compounds that have been central in the development of organometallic chemistry and homogeneous catalysis. Here, we evaluate how changes in the principal quantum number for the group IV (C5H5)2MCl2 (M = Ti, Zr, Hf; 1- 3, respectively) complexes affects the covalency of M-Cl bonds through application of Cl K-edge X-ray Absorption Spectroscopy (XAS). Spectra were recorded on solid samples dispersed as a thin film and encapsulated in polystyrene matrices to reliably minimize problems associated with X-ray self-absorption. The data show that XAS pre-edge intensities can be quantitatively reproduced when analytes are encapsulated in polystyrene. Cl K-edge XAS data show that covalency in M-Cl bonding changes in the order Ti > Zr > Hf and demonstrates that covalency slightly decreases with increasing principal quantum number in 1-3. The percent Cl 3p character was experimentally determined to be 26, 23, and 18% per M-Cl bond in the thin-film samples for 1-3 respectively and was indistinguishable from the polystyrene samples, which analyzed as 25, 25, and 19% for 1-3, respectively. To aid in interpretation of Cl K-edge XAS, 1-3 were also analyzed by ground-state and time-dependent density functional theory (TD-DFT) calculations. The calculated spectra and percent chlorine character are in close agreement with the experimental observations, and show 20, 18, and 17% Cl 3p character per M-Cl bond for 1-3, respectively. Polystyrene matrix encapsulation affords a convenient method to safely contain radioactive samples to extend our studies to include actinide elements, where both 5f and 6d orbitals are expected to play a role in M-Cl bonding and where transition assignments must rely on accurate theoretical calculations.

The dithiophosphinic acid HS(2)P(o-CF(3)C(6)H(4))(2) is known to exhibit exceptionally high extraction selectivities for trivalent minor actinides (Am and Cm) in the presence of trivalent lanthanides. To generate insight that may account for this observation, a series of [PPh(4)][S(2)PR(2)] complexes, where R = Me (1), Ph (2), p-CF(3)C(6)H(4) (3), m-CF(3)C(6)H(4) (4), o-CF(3)C(6)H(4) (5), o-MeC(6)H(4) (6), and o-MeOC(6)H(4) (7), have been investigated using sulfur K-edge X-ray absorption spectroscopy (XAS) and time-dependent density functional theory (TDDFT). The experimental analyses show distinct features in the spectrum of S(2)P(o-CF(3)C(6)H(4))(2)(-) (5) that are not present in the spectrum of 4, whose conjugate acid exhibits reduced selectivity, or in the spectra of 2 and 3, which are anticipated to have even lower separation factors based on previous studies. In contrast, the spectrum of 5 is similar to those of 6 and 7, despite the significantly different electron-donating properties associated with the o-CF(3), o-Me, and o-OMe substituents. The TDDFT calculations suggest that the distinct spectral features of 5-7 result from steric interactions due to the presence of the ortho substituents, which force the aryl groups to rotate around the P-C bonds and reduce the molecular symmetry from approximately C(2v) in 2-4 to C(2) in 5-7. As a consequence, the change in aryl group orientation appears to make the ortho-substituted S(2)PR(2)(-) anions "softer" extractants compared with analogous Ph-, p-CF(3)C(6)H(4)-, and m-CF(3)C(6)H(4)-containing ligands (2-4) by raising the energies of the sulfur valence orbitals and enhancing orbital mixing between the S(2)P molecular orbitals and the aryl groups bound to phosphorus. Overall, we report that sulfur K-edge XAS experiments and TDDFT calculations reveal unique electronic properties of the S(2)P(o-CF(3)C(6)H(4))(2)(-) anion in 5. These results correlate with the special extraction properties associated with HS(2)P(o-CF(3)C(6)H(4))(2), and suggest that ligand K-edge XAS and TDDFT can be used to guide separation efforts relevant to advanced fuel cycle development.