Synthesis, Structure, and Theoretical Calculations on NpO 2 Br 4 2 .

The actinide-halogen complexes (AnO 2 X 4 2- , X = Cl, Br, and I) are the simplest and most representative compounds for studying the bonding nature of actinides with ligands. In this work, we attempted to synthesize the crystals of NpO 2 X 4 2- (X = Cl, Br, and I). The crystals of NpO 2 Cl 4 2- and NpO 2 Br 4 2- were successfully synthesized, in which the structure of NpO 2 Br 4 2- was obtained for the first time. The crystal of NpO 2 I 4 2- could not be obtained due to the rapid reduction of Np(VI) to Np(V) by I - . The molecular structures of NpO 2 Cl 4 2- and NpO 2 Br 4 2- were characterized by single-crystal X-ray diffraction and infrared, Raman, and UV-Vis-NIR absorption spectroscopy. The complexes of NpO 2 X 4 2- (X = Cl, Br, and I) were also investigated by density functional theory calculations, and the calculated vibration frequencies and absorption features were comparable to the experimental results. Both the experimental results and theoretical calculations demonstrate the strengthened Np-O bonds and the weakened Np-X bonds across the NpO 2 X 4 2- series; however, the population analysis on the frontier molecular orbitals (MOs) of NpO 2 X 4 2- indicates a slight reduction in the Np-O bonding covalency and an enhancement in the Np-X bonding covalency from NpO 2 Cl 4 2- to NpO 2 I 4 2- . Results in this work have enriched the crystal database of the AnO 2 X 4 2- family and provided insights into the bonding nature in the actinide complexes with soft- and hard-donor ligands.