Carbon monoxide ($\mathrm{CO}$)

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.