Which of the following is not true about metal carbonyls?

The correct answer is Option (4) → pπ–pπ back bonding takes place

The metal–carbon bond in a metal carbonyl has σ and π character. A Lewis base donates an electron pair and a Lewis acid accepts one.

• σ bond (CO → M): the lone pair on the carbon of CO is donated into a vacant metal orbital.

• π bond (M → CO), back bonding: a pair of electrons from a filled d orbital of the metal is donated into the vacant antibonding π* orbital of CO, which is built from p orbitals. This is dπ–pπ overlap.

Option (1) CO acts as a Lewis base as well as a Lewis acid — True about metal carbonyls. CO donates its carbon lone pair (Lewis base) and accepts metal d electrons into its π* orbital (Lewis acid).

Option (2) The metal acts as a Lewis base as well as a Lewis acid — True about metal carbonyls. The metal accepts the lone pair of CO (Lewis acid) and donates its d electrons back to CO (Lewis base).

Option (3) dπ–pπ back bonding takes place — True about metal carbonyls. The back donation goes from a metal d orbital to the p-based π* orbital of CO.

Option (4) pπ–pπ back bonding takes place — Not true about metal carbonyls. The metal gives the back-bonding electrons from a d orbital, not a p orbital, so the overlap is dπ–pπ.

NCERT (2026-27 edition):
5.6 Bonding in Metal Carbonyls – “The metal-carbon bond in metal carbonyls possess both σ and π character. The M–C σ bond is formed by the donation of lone pair of electrons on the carbonyl carbon into a vacant orbital of the metal. The M–C π bond is formed by the donation of a pair of electrons from a filled d orbital of metal into the vacant antibonding π* orbital of carbon monoxide. The metal to ligand bonding creates a synergic effect which strengthens the bond between CO and the metal (Fig.5.14).”

Fig. 5.14: Example of synergic bonding interactions in a carbonyl complex