Temperature Dependence of Nuclear Quadrupole Resonance and the Observation of Metal–Ligand Covalency in Actinide Complexes: 35Cl in Cs2UO2Cl4

We report a study of the temperature dependence of 35Cl nuclear quadrupole resonance (NQR) transition energies and spin–lattice relaxation times (T 1) for 235U-depleted dicesium uranyl tetrachloride (Cs2UO2Cl4) aimed at elucidating electronic interactions between the uranium center and atoms in the equatorial plane of the UO2 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.