JEE Chemistry 2023 Question with Solution

Given: Assertion A states that in $\text{Ni(CO)}_4$ and $\text{Fe(CO)}_5$, 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.

The carbonyl ligand $\text{CO}$ is a neutral ligand. Therefore, in both $\text{Ni(CO)}_4$ and $\text{Fe(CO)}_5$, the oxidation state of the metal remains zero.

So, Assertion A is correct.

Low oxidation states of metals are stabilized by synergistic bonding or backbonding. This involves $\sigma$-donation from the ligand to the metal and $\pi$-backdonation from the metal to the ligand. For this, the ligand must have empty $\pi$ orbitals to accept electron density.

Thus, $\text{CO}$ acts as a $\pi$-acceptor ligand, not a $\pi$-donor ligand.

So, Reason R is not correct.

Therefore, the correct option is C.

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

Find: Whether Assertion A and Reason R are correct, and whether R explains A.

Assertion A: In $\text{Ni(CO)}_4$ and $\text{Fe(CO)}_5$, the carbonyl ligand $\text{CO}$ is neutral. Thus, the oxidation state of $\text{Ni}$ and $\text{Fe}$ in these complexes is indeed zero. So, Assertion A is correct.

Reason R: Low oxidation states of metals are stabilized by synergistic bonding, also known as backbonding. This involves $\sigma$-donation from the ligand to the metal and $\pi$-backdonation from the metal to the ligand. Crucially, the ligand must have empty $\pi$ orbitals to accept electrons from the metal. $\text{CO}$ is a $\pi$-acceptor ligand, not a $\pi$-donor ligand. So, Reason R is incorrect.

Therefore, the correct answer is (3): A is correct, but R is not correct.