Bonding Properties of Uranium Carbonate Species in Liquid and Solid State in Multiple Oxidation States Studied by u M4-Edge High Resolution X-Ray Spectroscopy

It is a long-standing discussion how the 5f electrons of the actinide (An) elements participate in chemical bonds. For investigating this and specifically the covalency of the An-ligand-bond [1, 2, 3], X-ray spectroscopy techniques, such as high-resolution x-ray absorption near edge structure (HR-XANES) [4] and core-to-core- and valence band resonant inelastic x-ray scattering (CC-/VB-RIXS) [5], are advanced tools. U M4-edge HR-XANES and VB-RIXS experiments were performed at the ACT station of the CAT-ACT beamline [7, 8] at the KIT Light Source, KIT, Germany [10].

In this study the changes in the electronic structure were investigated for uranium carbonate complexes with the oxidation states from IV to VI in liquid phase and U(VI) carbonate and nitrate solid phases. A spectro-electrochemical cell for simultaneous measurement of X-ray emission and UV-vis spectroscopy while applying a potential was successfully used to change in-situ the U oxidation state from VI to V while suppressing the disproportionation of U(V) [6].
A decrease in covalency of the O=U=O axial bond is detected using the HR-XANES spectra when reducing U(VI)-yl to U(V)-yl and U(V)-yl to U(IV). ... mehrWith a decreasing oxidation state of the uranium atom, the U 5f and 6p electron population of the occupied valence molecular orbitals also decreases as illustrated by the VB-RIXS spectra. This result suggests reduction of covalency for both axial and equatorial bonds.
[1] T. Vitova, et al., Nat. Commun. 2017, 16053.
[2] T. Vitova, et al., Inorg. 2020, 8-22.
[3] M. Zegke, et al., Chem. Sci. 2019, 9740-9751.
[7] A. Zimina, et al., Rev. Sci. Instrum. 2017, 113113.
[8] B. Schacherl, et al., J. Synchrotron Rad. 2022, 80-88.