Linearity and Chemical Bond of UO 2 2+ Revisited: A Comparison Study with UN 2 and UE 2 2+ (E = S, Se, and Te) Based on Relativistic Calculations.

The stability and electronic structure of UO 2 2+ are compared with those of UN 2 and UE 2 2+ (E = S, Se, and Te) based on four- and two-component relativistic Hamiltonians. We observed that the Hartree-Fock method overestimates the stability of the linear structures of UO 2 2+ and UN 2 . In addition to the conventional mechanism based on valence orbitals, we proposed another mechanism wherein the small energy difference between U's 6p 3/2 and O's σ(2s) orbitals destabilizes the bent structure of UO 2 2+ . The validity of the analysis based on the DFT method was evaluated using the coupled-cluster method. The slightly bent structures of UO 2 2+ and UN 2 are feasible from the viewpoint of energetic stability: the destabilized energy at 160° is 0.144 and 0.059 eV for UO 2 2+ and UN 2 , respectively. The U-X bond (X = N,O) is rigid in the slightly bent structure, and it corresponds to the conservation of the feature of the chemical bond. For UE 2 2+ , core-valence orbitals mainly affect the stability of these molecules, like UO 2 2+ and UN 2 . In UE 2 2+ , the 6p hole is fairly modest, and the 6p hole in UO 2 2+ is a unique feature in uranium-chalcogen systems.