Relativistic Density Functional Theory (DFT) based methods coupled with the Conductor-like Screening Model (COSMO) for a realistic solvation approach are used to investigate the electron affinity (EA) of a series of triscyclopentadienyl uranium complexes Cp 3 UX (X = Cl, BH 4 ,SPh,S i Pr and O i Pr) related to the U(IV)/U(III) redox system. E 1/2 half-wave potentials have been measured in solution (THF) under the same rigorous conditions for all the species under consideration. A good correlation (r 2 = 0.99) is found between the computed EA values, either in the gas phase or in solution, and the experimental half-wave potentials; the study brings to light the importance of spin-orbit coupling effects which must be taken into account in order to achieve the observed agreement between theory and experiment. The influence of the electron donating character of the X ligand on the orbital involved in the reduction process, namely the lowest unoccupied molecular orbital (LUMO) of the neutral U(IV) complexes, and on the EAs is discussed.

The redox behavior of tricyclopentadienyl- and phospholyluranium(IV) chloride complexes L(3)UCl with L = C(5)H(5) (Cp), C(5)H(4)Me (MeCp), C(5)H(4)SiMe(3) (TMSCp), C(5)H(4)(t)Bu ((t)BuCp), C(5)Me(5) (Cp*), and C(4)Me(4)P (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 L(3)UCl 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 [L(3)UCl](+) 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].