Electronic structures and magnetic properties of the U4+ complexes (C5Me4H)(3)UNO, (C5Me4H)(3)UCl, (C5H5)(3)UCH3, and (C5H5)(3)UCl are investigated by quantum chemical calculations. On the basis of wave function calculations including spinorbit (SO) interactions, all complexes have nondegenerate nonmagnetic ground states. However, for L = CH3 and Cl magnetic doublet excited states are very low in energy, rendering the magnetic susceptibility strongly temperature dependent above ca. 50-100 K. In contrast, (C5Me4H)(3)UNO exhibits temperature-independent paramagnetism even at room temperature. The calculated susceptibilities agree well with available experimental data. An analysis of the ground states and the magnetic behavior is performed using crystal-field (CF) models with parameters extracted from the ab initio calculations, and with the help of natural orbitals contributing to the electron density, generated from scalar relativistic and SO wave functions for the ground states and selected excited states. Electronic g-factors calculated from the CF models agree well with ab initio data. The UNO bond order in (C5Me4H)(3)UNO decreases somewhat due to SO coupling, because U-NO bonding p orbitals with strong U 5f(pi) character mix with nonbonding pi orbitals under the SO interaction. This complex also exhibits pronounced multireference character. All complexes afford U-ligand 5f covalent character.