Elucidation of the crystal structure of the dianionic complex [Na(18-crown-6)(thf)
2]
2[U(COT)(dddt)
2](COT =
8-C
8H
8, dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate), complementing that of [Na(18-crown-6)(thf)][U(COT)(dddt)
2], allowed the first structural comparison of analogous uranium(IV) and -(V)compounds with anionic sulfur ligands. The distinct conformations of the dithiolene ligands,
exo-
exoand
exo-
endo in the uranium (IV) and -(V) complexes, respectively, are the observable manifestation ofthe differences in the metal-ligand bonding according to the oxidation state of the metal. The relationshipbetween electronic structure, bonding, and conformational changes in these dianionic uranium(IV) andanionic uranium(V) species and the hypothetical neutral uranium(VI) counterpart has been investigatedin the framework of relativistic density functional theory. The calculations reveal the occurrence in theuranium(V) anionic species of a significant intramolecular interaction involving the 5f metal orbitals andthe C=C double bond of the
endo dithiolene ligand, which stabilizes its peculiar
exo-
endo conformation.Such an interaction does not exist in the uranium(IV) complex.