In a metal carbonyl, the M–C σ bond is formed by donation of the lone pair of electrons on the carbonyl carbon into a vacant orbital of the metal, and the M–C π bond is formed by donation of a pair of electrons from a filled d orbital of the metal into the vacant antibonding π* orbital of carbon monoxide. The strengthening of the bond between CO and the metal that results from this bonding is known as the
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Synergic effect
NCERT (2026-27 edition):
5.6 Bonding in Metal Carbonyls – “The metal-carbon bond in metal carbonyls possess both σ and π character. … The metal to ligand bonding creates a synergic effect which strengthens the bond between CO and the metal ….”