JEE Chemistry 2023 Question with Solution

Given: Assertion A states that in $\mathrm{Ni(CO)_4}$ and $\mathrm{Fe(CO)_5}$, the metals have zero oxidation state. Reason R states that low oxidation states are found when a complex has ligands capable of $\pi$-donor character in addition to the $\sigma$-bonding.

Find: Which option correctly evaluates Assertion A and Reason R.

For Assertion A, carbon monoxide is a neutral ligand. Therefore, in $\mathrm{Ni(CO)_4}$ and $\mathrm{Fe(CO)_5}$, the oxidation state of the metal remains $0$. So, Assertion A is correct.

For Reason R, low oxidation states are generally stabilized by ligands showing $\pi$-acceptor character along with $\sigma$-bonding, not by $\pi$-donor character. Carbon monoxide is a strong $\pi$-acceptor ligand. Hence, Reason R is not correct.

Therefore, the correct option is B: A is correct but R is not correct.

Given: The complexes are $\mathrm{Ni(CO)_4}$ and $\mathrm{Fe(CO)_5}$.

Find: Whether the stated reason explains the assertion.

1. In oxidation state calculation, each $\mathrm{CO}$ ligand contributes zero charge because it is a neutral ligand.
2. Hence,

and

So the assertion is true.

3. The stabilization of metals in low oxidation states occurs when ligands can accept electron density from the metal through back-bonding.
4. This requires $\pi$-acceptor ligands such as $\mathrm{CO}$, not $\pi$-donor ligands.

Therefore, the reason is false and does not explain the assertion. The correct option is B.