Infrared spectroscopy and structures of cobalt carbonyl cations, Co(CO)n+ (n = 1-9).

Cobalt carbonyl cations of the form Co(CO)(n)(+) (n = 1-9) are produced in a molecular beam by laser vaporization in a pulsed nozzle source. These ions, and their corresponding "argon-tagged" analogues, Co(CO)(n)(Ar)(m)(+), are studied with mass-selected infrared photodissociation spectroscopy in the carbonyl stretching region. The number of infrared-active bands, their frequency positions, and their relative intensities provide distinctive patterns allowing determination of the geometries and electronic structures of these complexes. Co(CO)(5)(+) has a completed coordination sphere, consistent with its expected 18-electron stability, and it has the same structure (D(3h) trigonal bipyramid) as its neutral isoelectronic analog Fe(CO)(5). The carbonyl stretches in Co(CO)(5)(+) are less red-shifted than those in Fe(CO)(5) because of charge-induced reduction in the pi back-bonding. Co(CO)(1-4)(+) complexes have triplet ground states, but the spin changes to a singlet for the Co(CO)(5)(+) complex.