Tricarbon monoxide C3O is a reactive radical oxocarbon molecule found in space, and which can be made as a transient substance in the laboratory.

[3] C3O has been detected by its microwave spectrum in the dark cold Taurus Molecular Cloud One[4] and also in the protostar Elias 18.

[5] The route to produce this is speculated to be:[6] or[5] The related C3S is more abundant in dark molecular clouds, even though oxygen is 20 times more common than sulfur.

[7] They produced carbon atoms by heating graphite inside a thin tantalum tube.

[13] Irradiating cyclopropenone with vacuum ultraviolet while frozen in a neon matrix causes dehydrogenation to form CCCO.

This is formed from [n-Bu4N][CrI(CO)5] and the silver acetylide derivative of sodium propiolate (AgC≡CCOONa), and then thiophosgene.

The complex can dissolve in hexane, however it slowly decomposes, losing dicarbon (C2) which goes on to form acetylenes and cumulenes in the solvent.

Dimethyl sulfoxide oxidises the CCCO ligand to carbon suboxide./[16] C3O deposits a reddish-black film on glass.

[17] The pathway for this, is that firstly the two molecules react to form isocyanuric acid and propiolamide, the NH then reacts to bond with the triple bond, with the NH2 group moving back.