The actinide-imide bonding revisited: A relativistic DFT study

The electronic structure of mono and bis(imide) cyclopentadienyl thorium and uranium complexes have been investigated using density functional theory (DFT) calculations in the framework of the relativistic zero order regular approximation (ZORA) implemented in the ADF (Amsterdam density functional) program. In the case of N-phenyl imido ligands, these complexes exhibit both σ and π interaction between the central metal atom and the three electron pairs borne by the nitrogen atom. These electron pairs interact strongly with the uranium orbitals, especially the 5f ones, insuring a great stability to these species. A triple bond is thus expected for the uranium-nitrogen interaction in these complexes. The geometry of the bis(imide) species is very peculiar. It is characterized by coplanar Nph phenylimide groups, very short U-N bond lengths and a linear U-N-Cph coordination angle. These features could not be observed for a transition metal complex, as this has been demonstrated on the example of molybdenum Mo(VI) species which exhibit a bent Mo-N-Cph coordination angle. We show that the 5f uranium orbitals are mainly responsible for this peculiar geometry.