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 35 Cl nuclear quadrupole resonance (NQR) transition energies and spin-lattice relaxation times ( T 1 ) for 235 U-depleted dicesium uranyl tetrachloride (Cs 2 UO 2 Cl 4 ) aimed at elucidating electronic interactions between the uranium center and atoms in the equatorial plane of the UO 2 2+ 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 T 1 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.