The Cc + CO reaction argon has been reinvestigated using deposition of ground-state reagents in solid argon. The Co + CO reaction products observed without activation energy are Co2CO, a van der Waals Co . . . CO complex, and the higher carbonyl species Co(CO)(x). Infrared radiation at approximately 3000 cm(-1), to promote Cc into the first excited-state b F-4 (3d(8)4s(1) configuration), convert,., the van der Waals complex to the chemically bound, most stable carbonyl form CoCO. A second irradiation with an approximately 580 nm wavelength (17000 cm(-1)) reconverts the chemically bound CoCO ((2)Delta) state into the Co . . . CO van der Waals ((4)Phi) state, The photoprocess is fully reversible. Isotopic data on nu (1), nu (2), nu (3), 2 nu (1), nu (1) + nu (2), and nu (1) + nu (3) have been measured in the near- and far-infrared regions for the carbonyl form. This enables a complete harmonic force-field calculation based on a linear geometry, in agreement with all theoretical predictions, DFT calculations of the geometrical and electronic properties of CoCO complexes from die Cc + CO reaction are also presented and compared to the experimental values. Reaction mechanisms are proposed and comparisons with bond force constants of NiCO and CuCO are also presented.