Vibrational spectroscopy of Ni+(benzene)n complexes in the gas phase

Ni+(benzene)(n) (n = 1-6) and Nil(benzene)(n)Ar-1.2 (n = 1,2) are produced by laser vaporization in a pulsed nozzle cluster source. The clusters are mass selected and studied by infrared laser photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer. The excitation laser is an OPO/OPA system that produces tunable IR in the C-H stretching region of benzene. Photodissociation of Nil(benzene), complexes occurs by the elimination of intact neutral benzene molecules, while Ni+(benzene)(n)Ar-1,Ar-2 complexes lose At. This process is enhanced on resonances, and the vibrational spectrum is obtained by monitoring the fragment yield versus the infrared wavelength. Vibrational bands in the 2700-3300 cm(-1) region are characteristic of the benzene molecular moiety with systematic shifts caused by the metal bonding. A dramatic change in the IR spectrum is seen at n = 3 and is attributed to the presence of external benzene molecules acting as solvent molecules in the cluster. The results of previous theoretical calculations are employed to investigate the structures, energetics, and vibrational frequencies of these complexes. The mono-benzene complex is found to have a C-2v, structure, with benzene distorted by the metal pi-bonding. The di-benzene complex is found to have a D-2h structure, with both benzenes distorted. The comparison between experiment and theory provides intriguing new insight into the bonding in these prototypical pi-bonded organometallic complexes.