Vibrational Spectroscopy and Density Functional Theory of Transition-Metal Ion−Benzene and Dibenzene Complexes in the Gas Phase

Metal−benzene complexes of the form M(benzene)n (M = Ti, V, Fe, Co, Ni) are produced in the
gas-phase environment of a molecular beam by laser vaporization in a pulsed nozzle cluster source. These
complexes are photoionized with an ArF excimer laser, producing the corresponding cations. The respective
mono- and dibenzene complex ions are isolated in an ion-trap mass spectrometer and studied with infrared
resonance enhanced multiple-photon dissociation (IR-REMPD) spectroscopy using a tunable free electron
laser. Photodissociation of all complexes occurs by the elimination of intact neutral benzene molecules,
and this process is enhanced on resonances in the vibrational spectrum, making it possible to measure
vibrational spectroscopy for size-selected complexes. Vibrational bands in the 600−1700 cm-1 region are
characteristic of the benzene molecular moiety with systematic shifts caused by the metal bonding. The
spectra in this solvent-free environment exhibit periodic trends in band shifts and intensities relative to the
free benzene molecule that varies with the metal. Density functional theory calculations are employed to
investigate the structures, energetics, and vibrational frequencies of these complexes. The comparison
between experiment and theory provides fascinating new insight into the bonding in these prototypical
organometallic complexes.