Terahertz dynamics at the actinide frontier: linking low-frequency vibrations, electronic structure, and bonding in molecular uranium complexes.

Understanding the nature of uranium-ligand bonding in molecular complexes is a central challenge in modern actinide chemistry. Here, we introduce low-frequency vibrational spectroscopy as a powerful new probe of bonding in uranium-containing molecular crystals by combining terahertz and low-frequency Raman spectroscopy with periodic solid-state density functional theory simulations. Using U(vi)(O t Bu)6 and its one-electron-reduced analogue as model systems, we show that uranium-oxygen bonding is encoded across the low-frequency spectrum through strongly mixed metal-ligand and ligand-lattice motions that cannot be interpreted using conventional gas-phase or high-frequency vibrational pictures alone. Local mode analysis isolates the intrinsic U-O stretching coordinates and reveals a clear weakening of the U-O bond upon reduction from U(vi) to U(v), while quantum theory of atoms in molecules (QTAIM) and natural bond order (NBO) analyses connect these vibrational changes to differences in electron density, orbital mixing, and covalency. Periodic calculations further show that crystal packing stiffens collective low-frequency motions in the neutral U(vi) lattice without significantly perturbing the underlying U-O bonding core, whereas the more charge-sensitive U(v) complex is strongly influenced by its ionic environment and associated non-covalent intermolecular interactions. These results establish, for the first time, a computationally validated correlation between low-frequency vibrational dynamics, local U-O force constants, and simulated bonding descriptors in molecular uranium complexes, and demonstrates the potential of this combined experimental and computational approach as a new route for interrogating electronic structure in heavy-element coordination complexes.