Anomalous Magnetism of Uranium(IV)-Oxo and -Imido Complexes Reveals Unusual Doubly- Degenerate Electronic Ground States

A fundamental part of characterising any metal complex is understanding its electronic ground state, for which magnetometry provides key insight. Most uranium(IV) complexes exhibit lowtemperature magnetic moments tending to zero, consistent with a non-degenerate spin-orbit ground state. However, there is a growing number of uranium(IV) complexes with low-temperature magnetic moments ³1 μB, suggesting a degenerate ground state, but the electronic structure implications and origins have been unclear. We report uranium(IV)-oxo and -imido complexes with low-temperature magnetic moments ca. 1.5-1.6 μB and show that they exhibit near-doubly degenerate spin-orbit ground states. We determine that this results from the strong point-chargelike donor properties of oxo and imido anions generating pseudo-symmetric electronic structures, and that traditional crystal field arguments are useful for understanding electronic structure and magnetic properties of uranium(IV). This suggests that a significant number of uranium(IV) complexes might benefit from a close re-evaluation of the nature of their spin-orbit ground states.