Co<sup>+</sup>(C<sub>2</sub>H<sub>2</sub>)<sub><i>n</i></sub> Complexes Studied\nwith Selected-Ion Infrared Spectroscopy\nand Theory

Co<sup>+</sup>(C<sub>2</sub>H<sub>2</sub>)<sub><i>n</i></sub> (<i>n</i> = 1–6) complexes produced\nwith\nlaser vaporization in a supersonic molecular beam are studied with\ninfrared photodissociation spectroscopy and computational chemistry.\nInfrared spectra are measured in the C–H stretching region\nusing the method of tagging with argon atoms to enhance the photodissociation\nyields. C–H stretch vibrations for all clusters studied are\nshifted to lower frequencies than those of the well-known acetylene\nvibrations from ligand → metal charge transfer interactions.\nThe magnitude of the red shifts decreases in the larger clusters as\nthe interaction is distributed over more ligands. Computational studies\nidentify various unreacted complexes with individual acetylene ligands\nin cation-π bonding configurations as well as reacted isomers\nin which ligand coupling reactions have taken place. Infrared spectra\nare consistent only with unreacted structures, even though the formation\nof reacted structures such as the metal ion-benzene complex is highly\nexothermic. Large activation barriers are predicted by theory along\nthe reaction coordinates for the <i>n</i> = 2 and 3 complexes,\nwhich inhibit reactions in these smaller clusters, and this situation\nis presumed to persist in larger clusters.