Uranium(III) compounds are very reactive and exhibit a broad range of chemical-bonding tendencies owing to the spatially diffused valence orbitals of uranium. A systematic study on the geometries, electronic structures, and chemical bonding of NU-XO (X = C, N, O) is performed using relativistic quantum chemistry approaches. The NU-CO and NU-NO complexes have an end-on structure, that is, (NU) (eta(1)-CO) and (NU) (eta(1)-NO), whereas NU-OO adopts a side-on ((NU) (eta(2)-O-2)) structure. The electronic structure analysis shows that UN exhibits efficient activation reactivity to molecules, especially to NO and O-2, because of the significant U 7s/5f -> XO 2 pi* electron transfer. Thus, the oxidation state of U is +V with the dianion ligand NO2- and O-2(2-) in NU-NO and NU-OO, respectively. Instead, U retains its usual +III oxidation state in NU-CO with a neutral CO ligand. The significant stability of NU-XO (X = C, N, 0) is determined by the covalent U-X bonding which contains both X -> U sigma-, pi-donation from the X lone pair and U 5f -> XO 2 pi* back-donation contributions. The significant back-donation to the antibonding X-O 2 pi* orbital results in the obvious weakening of the X-O bonding.