INFLUENCE OF THE NATURE OF THE LIGANDS ON THE ELECTRONIC GROUND-STATEOF ORGANOURANIUM(V) COMPOUNDS, STUDIED BY ELECTRON-PARAMAGNETIC-RESONANCE

Organouranium(V) amide compounds were studied by electron paramagnetic resonance in frozen solution. Their g-tensors were quantitatively int erpreted by assuming that dialkylamide, C8H8, C5H5, C5Me5, and THF lig ands interact only weakly with 5f orbitals of the central U(V) atom, s o that the total angular momentum J = 5/2 remains a good quantum numbe r for the description of the electronic ground state of these complexe s (weak-field approximation). In the absence of THF ligands, the groun d state is predominantly made of \M-J\ = 1/2 states, with a significan t admixture of \M-J\ = 5/2 states independent of the symmetry of the c omplex. Thus 5f orbitals are essentially nonbonding, and the metal lig and bonding should involve mainly uranium 6d orbitals. A THF ligand in duces a significant admixture of \MJ\ = 3/2 states. Substitution of am ide ligands by alkoxide ligands is characterized by strong U(V)(5f)-OR interactions, which breaks the weak-field approximation. This sensiti vity to oxygen ligands indicates that 5f orbitals contribute to the U( V)-oxygen bonding.