Inverse Trans Influenceand Uranium-Arene σ‑BondingDrive Molecular Geometry: Ligand Modification from Hard to Soft Flipsthe Oxide from Axial to Equatorial

A rare example of an equatorially bound terminal uranium(V) oxo complex in a chelating sulfur-based ligand environment, namely [(mes(^Me,AdArS)₃)U^V(O_eq)(THF)] (<b>2</b>), is presented. Octahedrally coordinated <b>2</b> is obtained by reaction of the mesitylene-anchored tris-thiophenolate-coordinated uranium(III) complex [U^III((SAr^Ad,Me)₃mes)] (<b>1</b>) with the oxygen-atom transfer reagent N₂O. The observed, equatorially bound oxo ligand in <b>2</b> is in stark contrast to its known tris-aryloxide analog, [(mes(^Me,AdArO)₃)U^V(O_ax)(THF)] (<b>A</b>), where the oxo ligand occupies the typically observed axial coordination site. Complexes <b>1</b> and <b>2</b> are characterized by single-crystal X-ray diffraction analyses and spectroscopic and magnetochemical methods, including ¹H NMR, UV/vis/NIR electronic absorption, as well as EPR spectroscopy and SQUID magnetometry, thus confirming the <i>C</i>_S symmetry and the pentavalent oxidation state of <b>2</b>. Encompassing quantum chemical calculations (DFT and CASPT2) on <b>2</b> and its tris-phenolate analog <b>A</b>, support and rationalize the structural and electronic differences. The molecular orbital pictures show that a stabilizing σ-bonding interaction arising from the U–O_eq inverse trans influence (ITI) is present in <b>2</b> but missing in <b>A</b>. In <b>2</b>, the sulfur 3p orbitals are closer in energy to the uranium 5f manifold than the arene π-system, leading to an ITI, while U–arene σ- or δ-bonding is not observed. Although the arene orbitals remain separated from the uranium 5f orbitals in <b>A</b>, the absence of an ITI allows the arene <i>a</i>_2u orbital to engage in a σ-type interaction with the metal. Thus, incorporating a tris-thiophenolate to an arene anchor introduces a new design concept in molecular f-element chemistry. This approach stabilizes an equatorially bound U(V) oxo center, contrasting with its tris-phenolate counterpart, where oxo coordination is axial. The observed geometric divergence, driven by competing ITI and U–arene interactions, not only tunes electronic structure but also leads to differentiated reactivity: only the phenolate analogs activate H₂O, while the thiolates do not.