Linearity and Chemical Bond of UO₂²⁺ Revisited: A Comparison Study with UN₂ and UE₂²⁺ (E = S, Se, and Te) Based on Relativistic Calculations

The stability and electronic structure of UO22+ are compared with those of UN2 and UE22+ (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 UO22+ and UN2. 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 sigma(2s) orbitals destabilizes the bent structure of UO22+. The validity of the analysis based on the DFT method was evaluated using the coupled-cluster method. The slightly bent structures of UO22+ and UN2 are feasible from the viewpoint of energetic stability: the destabilized energy at 160 degrees is 0.144 and 0.059 eV for UO22+ and UN2, 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 UE22+, core-valence orbitals mainly affect the stability of these molecules, like UO22+ and UN2. In UE22+, the 6p hole is fairly modest, and the 6p hole in UO22+ is a unique feature in uranium-chalcogen systems.