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Carbon monoxide is the diatomic compound $\mathrm{CO}$. Its strong carbon-oxygen bond and carbon-centered donor interaction make it an important ligand in metal carbonyls.

Its bonding to metal centers involves both donation from carbon monoxide and back-donation into its antibonding orbitals. This changes the carbon-oxygen stretching frequency, making infrared spectra a useful probe of a complex's electronic structure. Carbon monoxide is chemically distinct from carbon dioxide, despite containing the same two elements.

A metal carbonyl is a coordination complex containing carbon monoxide ligands. The ligand normally binds through carbon, donating an electron pair while accepting metal electron density into an antibonding orbital through pi backbonding.

These complementary interactions explain a useful spectroscopic test. Stronger backbonding tends to weaken the internal carbon–oxygen bond and lower its stretching frequency relative to weakly backbonded CO. A vibrational spectrum therefore probes electronic bonding, although terminal versus bridging coordination and the other ligands also affect the observed bands. Simple electron counting treats a neutral terminal CO as a two-electron donor, but that convention is not a complete description of bond strength.