Metal coordination converts the tub-shaped cyclo-octatetraene into anaromatic molecule: Electronic states and half-sandwich structures of groupIll metal-cyclo-octatetraene complexes

摘要

Group III (Sc, Y, and La) metal-(1,3,5,7-cyclo-octatetraene) (COT) complexes were producedin a laser-vaporization molecular beam source and studied by pulsed-field-ionizationzero-electron-kinetic-energy (ZEKE) spectroscopy. Adiabatic ionization energies and metal-ligandstretching frequencies were measured from the ZEKE spectra. Metal-ligand bonding and low-lyingelectronic states of the neutral and ionized complexes were analyzed by combining thespectroscopic measurements with the molecular orbital treatment and density functional theorycalculations. The ionization energies and metal-ligand stretching frequencies of these complexes arein the order of Sc > Y > La. The ground electronic state of the neutral complexes is 2A1, whereas theground state of the ions is 1/11. The molecular symmetry is C8? in both neutral and ionic groundstates. Although free COT is a nonaromatic molecule with a tublike structure, coordination of thegroup III metal atoms converts the tub-shaped molecule into a planar, aromatic structure. Thisconversion is induced by a two-electron transfer from the metal atoms to the ligand upon theformation of the complexes.