The burning of carbon in a limited supply of air or in a deficiency of oxygen produces carbon monoxide, e.g.,

Fe2O3  +  3C .....> 2Fe + 3CO

Carbon atom is considered to be sp hybridized. One sp orbital being utilised to form a single bond with oxygen atom while the other sp orbital which points away from the C-O bond contains a lone pair of electrons. The sideways overlap of the singly filled 2p orbital on carbon and oxygen atom produces a pi bond. The second pi bond is formed by the overlap of doubly filled 2p orbital on oxygen with the vacant 2p orbital on carbon. But once the bond is formed, it is not possible to distinguish the two pi bonds.

Because of the directed lone pair of electrons on carbon, the molecule forms carbonyls with a number of metals in which the coordinate bond is formed through carbon atom and not through oxygen atom.