The metal-carbon bond in metal carbonyls possess both s and p character.The M-C $\sigma$ bond is formed by the donation of lone pair of electrons onthe carbonyl carbon into a vacant orbital of the metal. The M-C $\pi$ bondis formed by the donation of a pair of electrons from a filled d-orbitalof metal into the vacant antibonding $\pi$* orbital of carbon monoxide. Themetal to ligand bonding creates a synergic effect which strengthens thebond between CO and the metal.