Electronic states and metal-ligand bonding of gadolinium complexes of benzene and cyclooctatetraene

Gadolinium (Gd) complexes of benzene (C₆H₆) and (1,3,5,7-cyclooctatetraene) (C₈H₈) were produced in a laser-vaporization supersonic molecular beam source and studied by single-photon pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy. Adiabatic ionization energies and metal-ligand stretching frequencies were measured for the first time from the ZEKE spectra. Metal-ligand bonding and electronic states of the neutral and cationic complexes were analyzed by combining the spectroscopic measurements with ab initio calculations. The ground states of Gd(C₆H₆) and [Gd(C₆H ₆)]⁺ were determined as ¹¹A₂ and ¹⁰A₂, respectively, with C_6v molecular symmetry. The ground states of Gd(C₈H₈) and [Gd(C ₈H₈)]⁺ were identified as ⁹A ₂ and ⁸A₂, respectively, with C_8v molecular symmetry. Although the metal-ligand bonding in Gd(C₆H ₆) is dominated by the covalent interaction, the bonding in Gd(C ₈H₈) is largely electrostatic. The bonding in the benzene complex is much weaker than that in the cyclooctatetraene species. The strong bonding in Gd(C₈H₈) arises from two-electron transfer from Gd to C₈H₈, which creates a strong charge-charge interaction and converts the tub-shaped ligand into a planar form. In both systems, Gd 4f orbitals are localized and play little role in the bonding, but they contribute to the high electron spin multiplicities.