(A) The metal-carbon bonds in metal carbonyls possess both σ and π-character. (B) The metal to ligand bonding creates a synergic effect which strengthens the bond between CO and the metal i.e. metal carbon bond becomes strong. (C) The metal-carbon σ is formed by the donation of lone pair of electrons on the carbonyl carbon into a vacant orbital of metal. (D) The metal-carbon π bond is formed by the donation of electrons from filled d-orbial of metal into vacant π* orbital of CO.