Metal coordination converts the tub-shaped cyclo-octatetraene into an aromatic molecule: Electronic states and half-sandwich structures of group III metal-cyclo-octatetraene complexes

Group III (Sc, Y, and La) metal-(1,3,5,7-cyclo-octatetraene) (COT) complexes were produced in a laser-vaporization molecular beam source and studied by pulsed-field-ionization zero-electron-kinetic-energy (ZEKE) spectroscopy. Adiabatic ionization energies and metal-ligand stretching frequencies were measured from the ZEKE spectra. Metal-ligand bonding and low-lying electronic states of the neutral and ionized complexes were analyzed by combining the spectroscopic measurements with the molecular orbital treatment and density functional theory calculations. The ionization energies and metal-ligand stretching frequencies of these complexes are in the order of Sc>Y>La The ground electronic state of the neutral complexes is ²A₁, whereas the ground state of the ions is ¹A₁. The molecular symmetry is C_8ν in both neutral and ionic ground states. Although free COT is a nonaromatic molecule with a tublike structure, coordination of the group III metal atoms converts the tub-shaped molecule into a planar, aromatic structure. This conversion is induced by a two-electron transfer from the metal atoms to the ligand upon the formation of the complexes.