Do the ``Ligand Field'' Parameters in Lanthanides Represent Weak Covalent Bonding?

Instead of explaining the seven different f-orbital energies or five different d-orbital energies by parameters Anm〈rn〉 of the electrostatic ligand field model, we propose to classify the energy levels according to the actual one-electron energies and to interpret these quantities by the weak effects of σ antibonding on the partly filled shell. Calculations of the relative angular dependence Ξ2 of such effects are made in a simple model and compared with experimental data for nine- and eight-coordinated lanthanide compounds. The agreement is judged to be much more satisfactory than when the electrostatic model is applied, and the number of freely chosen parameters is much smaller.