Metal carbonyls are compounds in which carbon monoxide (CO) molecules are coordinated to a transition metal through metal–carbon bonds. The bonding in metal carbonyls is explained by the concept of synergic bonding, which involves two complementary interactions: σ-donation from carbon monoxide to the metal and π-back donation from the metal to carbon monoxide. These two interactions reinforce each other and produce a strong metal–CO bond. CO has a carbon atom with a lone pair of electrons present in a filled molecular orbital. This orbital is directed along the internuclear axis and can donate its electron pair to an empty or partially empty orbital of the metal. This interaction is called σ-donation. A simple representation is: ```text :C≡O │ │ σ donation ↓ M ``` The carbon atom of CO acts as the donor atom because its lone pair is concentrated mainly on carbon. The donation of this electron pair forms a coordinate σ bond between the metal and carbon. However, σ-donation alone does not adequately explain the strength and stability of metal carbonyls. CO also possesses low-energy empty π*