Explanation

[Solution from source] Back-donation from metal d orbitals into CO π* orbitals weakens the C–O bond. The positive charge in Mn(CO)₆⁺ holds the metal electrons tightly, so back-bonding is least and the C–O bond is strongest. The anion V(CO)₆⁻ has the most back-bonding and the weakest C–O bond. Answer (3).