Determining Relative f and d Orbital Contributions to M–Cl Covalency in MCl₆^2– (M = Ti, Zr, Hf, U) and UOCl₅^– Using Cl K-Edge X-ray Absorption Spectroscopy and Time-Dependent Density Functional Theory

Abstract

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.

Covalency in Metal–Oxygen Multiple Bonds Evaluated Using Oxygen K-edge Spectroscopy and Electronic Structure Theory

“…Analogous increases in pre-edge peak energy have been observed previously in the Cl and S K-edge XAS of transition metal coordination compounds, and were attributed to the increasing energy of the 3d, 4d, and 5d atomic orbitals. [25][26][27][28][29] Group 7 d 0 KMnO 4 , (Ph 4 P)TcO 4 , and NaReO 4 salts were analyzed to evaluate how traversing from left to right in the periodic table affected the O K-edge MO 4 x-spectra (Supporting Information). Not all measurements could be performed on KMnO 4 or (Ph 4 P)TcO 4 owing to the susceptibility of permanganate towards radiation damage and difficulties associated with handling radioactive 99 Tc.…”

“…53 These functionals and basis sets have demonstrated good agreement between experimental and simulated ligand K-edge XAS for organometallic and inorganic systems. 21,26,29,54 The molecular orbital compositions of each compound were obtained by Mulliken population analysis of the individual molecular orbitals.The O K-edge XAS were simulated using TDDFT. This approach has been applied successfully to simulate spectra for several other transition metal systems 26,27,[34][35][36] and to simulate the absorption spectrum of permanganate.…”

A Macrocyclic Chelator That Selectively Binds Ln4+ over Ln3+ by a Factor of 1029

“…[173][174][175] In this model, the 4f orbitals for Figure 13. Qualitative molecular orbital diagram depicting the interaction between lanthanide 4f or actinide 5f orbitals and ligand 2p or 3p orbitals to form a metal-ligand bond.…”

“…It is important to note that the energetic stabilization gained due to 4f covalency is proportional to the overlap between 4f and ligand-based orbitals. 173,175 Hence, because the radial extension of the 4f orbitals is very small, [176][177][178][179] increasing 4f covalency by decreasing the energy separation of the metal and ligand orbitals may act to weaken the bond overall by decreasing the amount of ionic character in the ground state. This work also illustrates that thermodynamic, spectroscopic, and theoretical studies of tetravalent cerium molecules is a fruitful area for research.…”

Dual roles of f electrons in mixing Al 3p character into d -orbital conduction bands for lanthanide and actinide dialuminides

“…Al is their energy separation [65,66]. For EuAl 2 and YbAl 2 , contributions from the divalent configuration (Ln 2+ , 4f n+1 ) are increased relative to the normal trivalent configuration (Ln 3+ , 4f n ).…”