Half-sandwich structure of cyclopentadienyl dialuminum [Al2(eta5-C5H5)] from pulsed-field ionization electron spectroscopy and ab initio calculations.

Cyclopentadienyl dialuminum [Al2Cp, Cp = C5H5] was prepared in a pulsed laser ablation cluster beam source and identified with a time-of-flight photoionization mass spectrometer. The high-resolution electron spectrum of this complex was obtained using pulsed-field ionization zero electron kinetic energy (ZEKE) photoelectron spectroscopy. Three isomeric structures with two Al atoms residing on the same or opposite sites of the Cp plane were predicted by second-order Møller-Plesset perturbation theory. A half-sandwich structure with an aluminum dimer perpendicular to the Cp plane was identified by the experiment. The ground electronic states of the neutral and ionized species are 2A' ' in Cs symmetry and 1A1 in C5v symmetry, respectively. In both the neutral and ionic states, one of the Al2 atoms binds with five carbons, and the metal-ligand bonding consists of orbital and electrostatic contributions. Ionization of the 2A' ' neutral state enhances the metal-ligand bonding but weakens the metal-metal interaction.