Reactions of laser-ablated scandium and yttrium atoms with dilute carbon monoxide molecules in solid argon have been investigated using matrix-isolation infrared spectroscopy. On the basis of the results of the isotopic substitution, the change of laser power and CO concentration and the comparison with density functional theory (DFT) calculations, the absorption at 1193.4 cm(-1) is assigned to the C-O stretching vibration of the Sc-2[eta(2)(mu(2)-C, O)] molecule, which has a single bridging CO that is tilted to the side. This CO-activated molecule undergoes ultraviolet-visible photoinduced rearrangement to the CO-dissociated molecule, c-Sc-2(mu-C)(mu-O). The cyclic c-Sc-2(mu-C)(mu-O) molecule has a bridging carbon on one side of the Sc2 unit and a bridging oxygen on the other. The analogous Y-2[eta(2)(mu(2)-C,O)] molecule has not been observed, but the CO-dissociated c-Y-2(mu-C)(mu O) molecule has been observed in the Y + CO experiments. DFT calculations of the geometry structures, vibrational frequencies, and IR intensities strongly support the assignments. The CO activation mechanism has also been discussed. Our experimental and theoretical results schematically depict an activation process to CO dissociation.