A Molecular Orbital Study of Bonding and Ionization Energies in Pentavalent Uranium Imide/Amide Complexes

Discrete variational local density functional (Xα) calculations on the model complexes [(RN)U(NR₂)₃] (R = H (1), SiH₃(2)) have been performed in order to investigate the electronic structure of uranium amido/imido complexes. Special emphasis is placed on the energy ordering of the highest lying levels formed by U 5f orbitals and by out-of-plane N 2p_πorbitals. Orbital relaxations are also estimated by studying the fragments NR and NR₂with R = H and R = SiMe₃in order to allow a comparison of the model compounds 1 and 2 with the measured photoelectron spectrum of the complex |(Me₃SiN)U[N(SiMe₃)₂]₃). The ionization energies corresponding to the N 2p_πMOs are calculated in the order N 2p_π(amido) < N 2p_π(imido). The calculated level ordering is rationalized by comparing the order of the two types of U-N bonds and the charge distributions of the corresponding ligands. Taking the calculated ordering as well as the character of the molecular orbitals into account, a new assignment of the observed photoelectron spectra is given based on a comparison of He I and He II spectra.