Equatorial Ligation Controls 5f Electron Binding in Penta- and Hexavalent Uranyl Acetate Complexes

Anion photoelectron spectroscopy (aPES), in conjunction with relativistic electronic-structure calculations, probed the electronic structures of the di- and triacetate uranyl complex anions, (AcO)₂UO₂⁻ and (AcO)₃UO₂⁻. These anions were generated by electrospray and photodetached at 355 nm for the diacetate and 193 nm for both species. The experimentally assigned vertical electron detachment energies are 2.77 and 6.02 eV for the di- and triacetate complexes, respectively. Electronic-structure calculations were performed to reproduce the aPES spectra. Wave function analysis indicates a UV(5f¹) center for (AcO)₂UO₂⁻ and a UVI(5f⁰) center for (AcO)₃UO₂⁻. Therefore, the detached electron is the 5f¹ electron of the former, which leads to significant spin-orbit effects, while for the latter, the detached electron comes from an acetate oxygen. In both systems, all U-O bonds are highly ionic with minimal covalent character. Our experimental measurements in tandem with our computational findings highlight how higher coordination numbers can adjust the degree of 5f electron participation in bonding and increase the stabilization of the anion.