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 (Oh ) symmetry and a doublet X2 A2u (Yb) and singlet X1 A1g (Lu) electronic ground state, whereas Tm(CO)8 - exhibits a D4h equilibrium geometry and a triplet X3 B1g 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.