Quantum Chemistry Study of Actinide(III) and Lanthanide(III) Complexes with Tridentate Nitrogen Ligands

Abstract

The structure and bonding in large complexes of actinide(III) and lanthanide(III) with tridentate N-donor ligands and water molecules have been investigated through quantum chemistry calculations in order to characterize the nature of the lanthanide−ligand and actinide−ligand bonds. Calculations have been performed using relativistic density functional theory on [M(L)(H2O)6]3+, [M(L)(H2O)5Cl]2+, and [M(H2O)9]3+ clusters where M = La, Ce, Nd, U, Pu, Am, or Cm and L = 2,2‘:6‘2‘ ‘terpyridine (Terpy) or 2,6-bis(5,6-dimethyl-1,2,4-triazin-3-yl)pyridine (MeBtp). Calculated M−L distances compare well with X-ray crystal data obtained on related systems. In particular, calculations correctly reproduce the experimentally observed shortening of the uranium−ligand bond in comparison with the cerium−ligand bond. The calculated evolution of the M−L bond as a function of the cation shows that lanthanide−ligand distances decrease with the diminution of the ionic radius, whereas the actinide−ligand distances increase from uranium to americium and are shorter than Ln−N distances. These trends are explained by the presence of slightly stronger covalent effects in the metal−ligand bond for the actinides, decreasing in the order U > Pu > Am ≈ Cm, compared to lanthanides. The participation of 5f orbitals in the bonding is found to be significant only for uranium.