Question

The bond length of $M-C$ bond in metal carbonyls after back-bonding:

• A. Increases.
• B. Cannot be predicted.
• C. Remains constant.
• D. Decreases.

Answer 1

The correct option is D Decreases.

Synergic bonding interactions are present in metal carbonyls. The sigma bond is formed when lone pair of electrons of carbonyl carbon is donated to vacant orbital of metal. To form a pi bond, an electron pair donated from filled metal d orbital to vacant anti bonding pi orbital of $CO$.This involves lateral overlap of orbitals. Due to this, the bond order of $M-C$ increases and bond order of $CO$ decrease.
We know,
$\text{Bond length}\propto \frac{1}{\text{Bond order}}$

Therefore, bond length of $M-C$ decreases and of $C-O$ increases.

Hence, option (b) is correct.