Compounds in which a metal directly bonded to a carbon atom are called organometallic compounds. Metal carbonyls are a class of organometallic compounds in which the metal connected to a carbonyl ligand through coordinate bond. The sigma bond between metal and carbon will be generated by the donation of a pair of electron by the carbon to the vacant orbital of metal. If the metal has high electron density around it, there is a possibility that the metal can do a pi-back bonding by putting a pair of electron to the vacant pi* orbital of carbonyl ligand. This pi back bonding increases the stability of the metal carbonyl bond. At the same time it reduces the bond order between carbon and oxygen. Both sigma and pi donations occur simultaneously to achieve greater stability and we call it as synergic bonding.

Watch the video to understand more about synergic bonding