Octacarbonyl Anion Complexes of the Late Lanthanides Ln(CO) ₈ ⁻ (Ln=Tm, Yb, Lu) and the 32-Electron Rule

The lanthanide octacarbonyl anion complexes Ln(CO) ₈ ⁻ (Ln=Tm, Yb, Lu) were produced in the gas phase and detected by mass-selected infrared photodissociation spectroscopy in the carbonyl stretching-frequency region. By comparison of the experimental CO-stretching frequencies with calculated data, which are strongly red-shifted with respect to free CO, the Yb(CO) ₈ ⁻ and Lu(CO) ₈ ⁻ complexes were determined to possess octahedral (O _h ) symmetry and a doublet X ² A _2u (Yb) and singlet X ¹ A _1g (Lu) electronic ground state, whereas Tm(CO) ₈ ⁻ exhibits a D _4h equilibrium geometry and a triplet X ³ B _1g ground state. The analysis of the electronic structures revealed that the metal-CO attractive forces come mainly from covalent orbital interactions, which are dominated by [Ln(d)]→(CO) ₈ π backdonation and [Ln(d)]←(CO) ₈ σ donation (contributes ≈77 and 16 % to covalent bonding, respectively). The metal f orbitals play a very minor role in the bonding. The electronic structure of all three lanthanide complexes obeys the 32-electron rule if only those electrons that occupy the valence orbitals of the metal are considered.