Dicarbon monoxide (C2O) is an extremely reactive molecule that contains two carbon atoms and one oxygen atom. It is covalently bonded. It is a product of the photolysis of carbon suboxide. It is closely related to CO, CO2 and C3O2, and other oxocarbons.
Dichlorine monoxide, Cl2O, is a chlorine oxide. It is a brownish yellow gas at room temperature which can explode in high concentrations when heated or sparked.