Investigating the electronic structure and bonding in uranyl compounds by combining NEXAFS spectroscopy and quantum chemistry

The nature of the reactivity of the “yl” oxygens has been a subject of constant interest for a long time in uranyl chemistry. Thus, the electron-donor ability of the equatorial ligands plays an important role in the nature of the uranyl U=O bond. In this paper, a combination of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and both ground-state and time-dependent density functional theory (DFT) calculations have been used to examine the effect of equatorial plane ligation on the U=O bonding in two uranyl complexes: [UO₂(py)₃I₂] and [UO₂(CN)₅][NEt₄]₃. By coupling experimental data and theory, spectral features observed in the oxygen K-edge NEXAFS spectra have been assigned. Despite the inert character of the U=O bond, we observe that the electron-donating or withdrawing character of the equatorial ligands has a measurable effect on features in the NEXAFS spectra of these species and thereby on the unoccupied molecular orbitals of (UO₂)²⁺.