For carbonyl as a ligand, see Metal carbonyl. A compound containing a carbonyl group (C=O). In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O.It is common to several classes of organic compounds, as part of many larger functional groups.A compound containing a carbonyl group is often referred to as a carbonyl compound.or carbonyl as a ligand, see Metal carbonyl. A compound containing a carbonyl group (C=O). In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O.