The redox behavior of tricyclopentadienyl- and phospholyluranium(IV) chloride complexes L3UCl with L = C5H5 (Cp), C5H4Me (MeCp), C5H4SiMe3 (TMSCp), (C5H4Bu)-Bu-t ((BuCp)-Bu-t), C5Me5 (Cp*), and C4Me4P (tmp), has been investigated using relativistic density functional theory calculations, with the solvent being taken into account using the conductor-like screening model. A very good linear correlation (r(2) = 0.99) has been obtained between the computed electron affinities of the L3UCl complexes and the experimental half-wave reduction potentials E-1/2 related to the U-IV/U-III redox systems. From a computational point of view, our study confirms the crucial importance of spin orbit coupling and solvent corrections and the use of an extended basis set in order to achieve the best experiment theory agreement. Considering oxidation of the uranium(IV) complexes, the instability of the uranium(V) derivatives [L3UCl](+) is revealed, in agreement with experimental electrochemical findings. The driving roles of both the electron-donating ability of the L ligand and the U 5f orbitals on the redox properties of the complexes are brought to light. Interestingly, we found and explained the excellent correlation between variations of the uranium Hirschfeld charges following U-IV/U-III electron capture and E-1/2. In addition, this work allowed one to estimate theoretically the half-wave reduction potential of [Cp-3*UCl].