Which of the following statements is true regarding the bonding in metal carbonyls?

Solution

1. Metal carbonyls exhibit synergic bonding involving $\sigma$-donation from $\text{CO}$ to metal and $\pi$-back-donation from metal to $\text{CO}$.
2. The $\pi$-back-donation populates the antibonding ${\pi }^{\ast }$ orbitals of the $\text{CO}$ ligand.
3. Populating antibonding orbitals reduces the bond order of the $\text{C-O}$ bond.
4. This simultaneously increases the $\text{M-C}$ bond order.
5. Hence the answer is (C).