Investigation of the dithiolene ligand conformation in analogous U(IV)/U(V) complexes:: X-ray diffraction and density functional theory analysis of the U•••(C=C) interaction

Elucidation of the crystal structure of the dianionic complex [Na(18-crown-6)(thf)(2)](2)[U(COT)(dddt)(2)] (COT = eta(8)-C8H8, 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-exo and exo-endo in the uranium (IV) and -(V) complexes, respectively, are the observable manifestation of the differences in the metal-ligand bonding according to the oxidation state of the metal. The relationship between electronic structure, bonding, and conformational changes in these dianionic uranium(IV) and anionic uranium(V) species and the hypothetical neutral uranium(VI) counterpart has been investigated in the framework of relativistic density functional theory. The calculations reveal the occurrence in the uranium(V) anionic species of a significant intramolecular interaction involving the 5f metal orbitals and the 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.