Laser-ablated ruthenium and osmium atoms, cations and electrons have been reacted with CO molecules during condensation in excess neon. The Ru(CO)x and Os(CO)x (x = 1−5) molecules are formed during sample deposition or on annealing. The Ru(CO)x- and Os(CO)x- (x = 1−4) anions are formed by electron capture, and the RuCO+ and OsCO+ cations are produced by metal cation reactions and identified from 13CO and C18O isotopic substitution, density functional calculations, and behavior on doping with CCl4 to serve as an electron trap. Density functional calculations predict these frequencies within 1% and the isotopic shifts for different C−O stretching modes within averages of 1 to 2 cm-1 including vibrational modes for both C3v and D2d tetracarbonyl anion structures.