Covalency in the uranyl ion: A polarized x-ray spectroscopic study

Abstract

Polarized oxygen Kα x-ray absorption and emission spectra, near 530 eV, of a single crystal of Cs2UO2Cl4 are reported. With the aid of density functional theory calculations and the absorption data, the relative energies of the empty molecular orbitals having primarily uranium 5f and 6d character are assigned. The emission spectra give access to the energy of excitations to these orbitals from the various filled valence-shell orbitals. These energies support the conclusion from the optical spectra that valence excitations from the σu occupied valence orbitals occur at substantially lower energies than those from the σg, πg, and πu orbitals. This latter group of molecular orbitals have much larger oxygen-2p character. The participation of the pseudocore 6p shell in the covalent bonding is established directly by the presence of a charge-transfer transition in emission. With the aid of previous work on the polarized uranium L1- and L3-edge absorption spectra, the order of the empty metal-centered antibonding valence orbitals is established as 5fπ<5fσ∼6dδ∼6dσ<6dπ. The greater antibonding character of 6dπ compared with 6dσ can be explained by the relatively small overlap of the latter orbital with oxygen 2p. However, the relative energies of 5fπ and 5fσ cannot be due to differences in overlap and are attributed instead to a strong hybridization between 5fσ and 6pσ. This superposition is confirmed by the linear dichroism in the U L1-edge absorption spectrum.

Combining theoretical chemistry and XANES multi-edge experiments to probe actinide valence states

“…In this diagram, only the transitions to levels corresponding to uranium 7s, 6d and 5f orbitals hybridized with oxygen 2p orbital are authorized in oxygen K edge absorption. The order of the empty anti-bonding valence orbitals is established as p u * (5f p ) < s g *(6d s ) < s u *(5f s ) < p g *(6d p ) and is the same as the one found by Denning et al [2] with the presence of a 1s core hole (Fig. 12).…”

“…Oxygen K edge is an excellent probe of the covalency of the actinideeoxygen bond, since the actinide 5f/6d and O 2p orbitals form an anti-bonding orbital [2]. Furthermore, those edges are very sensitive to the oxygen 2p states because of the very small core-hole width of oxygen (0.2 eV).…”

“…In this field, the actinide L 3 edge (corresponding to the dipolar 2pe6d transition) has been most frequently used because the energy range involved (from 16 to 19 keV) is consistent with the presence of radiological barriers around the sample. Although significant structural information on the coordination polyhedron can be obtained from these electronic transitions because of the importance of shape resonances [2,3], the very short core-hole lifetime broadens the edge signal (about 7 eV for U) resulting in very little extractable electronic information. In most studies, a systematic investigation of the relative L 3 edge position and width results in a qualitative comparison of the metal environment and its influence on the cation effective charge as well as the influence of the ligands.…”

Closure of the Mott gap and formation of a superthermal metal in the Fröhlich-type nonequilibrium polaron Bose-Einstein condensate in UO2+x

“…Figure 1A shows the O XAS of: UO 2 that is cubic with U(IV); U 3 O 8 that is layered with two distinct U(V) and (VI) sites; α UO 3 that is layered and U(VI) [31][32][33][34] ; and the molecular complex Cs 2 UO 2 Cl 4 that possesses the U(VI) trans dioxo moiety that is inherent to the higher U valences and defines the oblate local geometry that is the basis for layering. 35 Because of the symmetry relationship between the cubic U site of UO 2 and the quasi-octahedral site of the layered materials and uranyl complexes these spectra for both geometries consist of two peaks. The lower energy peak is of predominantly U 5f character, and is well separated and distinct from a higher energy peak that is predominantly of U 6d character.…”

Actinides in Solution: Complexation and Kinetics

“…The most prevalent example of covalence in actinide bonding comes from actinide-ligand multiple bonds (Kaltsoyannis, 2000;Denning et al, 2002), particularly the short (ca. 1.7-1.8 Å ) O¼An¼O bonds in the linear dioxo actinyl ions of the pentavalent and hexavalent light actinides, AnO þ 2 and AnO 2þ 2 ðAn ¼ U; Np; Pu; AmÞ.…”