Theoretical Investigations of Geometry, Electronic Structure and Stability of UO₆: Octahedral Uranium Hexoxide and Its Isomers^†

Abstract

The existence of a novel octahedral UO(6) complex had been suggested by Pyykko et al. [Pyykko, P.; Runeberg, N.; Straka, M.; Dyall, K. G. Chem. Phys. Lett. 2000, 328, 415]. We have now investigated the stability, the geometric and electronic structures, and the vibrations of various UO(6) molecules, using spin-orbit density functional and scalar-relativistic coupled-cluster approaches. We find four different (meta-)stable species, namely (3)D(2h)-UO(2)(eta(2)-O(2)(*))(2) at lowest energy, (3)C(2v)-UO(4)(*)(eta(2)-O(2)(*)) and (1)D(3)-U(eta(2)-O(2))(3) at medium energies, and (1)O(h)-UO(6) at highest energy. The decay of O(h)-UO(6) occurs via an activated spin-flip mechanism. The UO(6) species correspond to local minima on singlet and triplet energy surfaces and might be trapped in noble gas matrices. Experimentally, the four species might be identified through their vibrational spectra. Uranium is best described as coordinated by oxygen atoms in various oxidation states as oxo O(2-), oxido(1) O(*-), peroxido O(2)(2-), and superoxido O(2)(*-) ligands. The occurrence of monovalent oxygen is remarkable. The resulting characterization of the central ion as U(VI) in all four cases does not fully reflect the electronic differences, nor the "valence-activity" of the U-6p(6) semicore shell.

Oxygen radical character in group 11 oxygen fluorides

“…As pointed out by a referee we noticed that such a breakdown of the conventional Lewis electron–pair model is not a unique phenomenon of group 11 oxygen compounds. One closely related example is the recently investigated oxygen-centered OHgF radical 39 , but also other transition metal terminal oxo complexes bearing terminal oxygen-centered radicals have been predicted 22 – 24 , 40 . Terminal oxygen radicals have also been found to occur in metal oxide clusters of early transition metals, when these metals are in their highest oxidation state.…”

“…This study gives insight into the nature of the unusual group 11 metal-oxygen bond and proves the radical-like character of the oxo ligands in these complexes. Because of the non-innocent character of the terminal oxo ligand, assigning metal oxidation states in these complexes becomes ambiguous 22 – 24 .…”

Probing the Electronic Structure and Chemical Bonding in Tricoordinate Uranyl Complexes UO2X3– (X = F, Cl, Br, I): Competition between Coulomb Repulsion and U–X Bonding

“…,77 On the other hand, SO effects are negligible for the F 2p-type orbitals, while the 5e′ U−O bonding orbital in UO 2 F 3 − exhibits a large SO effect.…”

Diversity of Chemical Bonding and Oxidation States in MS4 Molecules of Group 8 Elements

“…An extreme example is (metastable) spin‐singlet O h ‐UO 6 =(U 6+ )( . O 1− ) 6 , in which the single electrons on the O atoms are coupled with each other and through the empty U 5f6d7s valence shell . The published cases of FU: –:SiH, C .…”