In metal carbonyls, M-C pi bond is formed by :
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Donation of a pair of electrons from a filled d-orbital of metal into the vacant anti-bonding orbital of carbon monoxide.
The π part of the M–C bond is formed by back bonding from the metal to CO. Since the back-donated electrons enter an antibonding orbital of CO, the C–O bond becomes slightly weaker.
So the M–C π bond is formed by donation from a filled metal d orbital into the vacant antibonding orbital of CO.
Why the other options are incorrect
Option (2) Donation of a pair of electrons from a filled d-orbital of metal into the vacant bonding orbital of carbon monoxide — Incorrect. The bonding orbitals of CO are already filled, so they cannot accept electrons. Only the antibonding π* orbitals are vacant.
Option (3) Donation of a pair of electrons from a filled p-orbital of metal into the vacant anti-bonding orbital of carbon monoxide — Incorrect. Back bonding uses the filled (n−1)d orbitals of the transition metal, not p orbitals.
Option (4) Donation of a pair of electrons from anti-bonding orbital of carbon monoxide into the vacant d-orbital of metal — Incorrect. The antibonding orbital of CO is empty, so it has no electrons to donate. Donation from CO to the metal comes from the carbon lone pair and forms the σ bond.
NCERT (2026-27 edition):
Bonding in Metal Carbonyls – …
