Temperature Dependence of Nuclear Quadrupole Resonance and the Observation of Metal-Ligand Covalency in Actinide Complexes: 35 Cl in Cs 2 UO 2 Cl 4 .

We report a study of the temperature dependence of Cl nuclear quadrupole resonance (NQR) transition energies and spin-lattice relaxation times () for U-depleted dicesium uranyl tetrachloride (CsUOCl) aimed at elucidating electronic interactions between the uranium center and atoms in the equatorial plane of the UO ion. The transition frequency decreases slowly with temperature below 75 K and with a more rapid linear dependence above this temperature. The spin-lattice relaxation time becomes shorter with temperature, and as temperatures increase, the decrease becomes nearly quadratic. The observed trends are reproduced by a model that assumes phonon-induced fluctuations of the electric field gradient tensor and partial electron delocalization from Cl to U. The fit of the theoretical model to experimental data allows a Debye temperature of 96 K to be estimated. The generalization of this approach to investigations of covalency in actinide-ligand bonding is examined.