Reactions of silicon atoms and small clusters with carbon monoxide molecules in solid argon have been studied using matrix isolation infrared absorption spectroscopy. In addition to the previously reported SiCO monocarbonyl, Si-2(CO)(2) and SinCO (n=2-5) carbonyl molecules were formed spontaneously on annealing and were characterized on the basis of isotopic substitution and theoretical calculations. It was found that Si2CO, Si3CO, and Si5CO are bridge-bonded carbonyl compounds, whereas Si4CO is a terminal-bonded carbonyl molecule. The Si-2(CO)(2) and Si3CO molecules photochemically rearranged to the more stable c-Si-2(mu-O)(mu-CCO) and c-Si-2(mu-O) (mu-CSi) isomers where Si-2 is inserted into the CO triple bond. (C) 2004 American Institute of Physics.