Check Assertion A

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

Carbon monoxide, $\mathrm{CO}$, is a neutral ligand.

Let the oxidation state of the metal be $x$.

• For $\mathrm{Ni(CO)_4}$: $x + 4(0) = 0 \implies x=0$

• For $\mathrm{Fe(CO)_5}$: $x + 5(0) = 0 \implies x=0$

Hence, in both complexes the metals are in zero oxidation state.

So, Assertion A is correct.

Check Reason R

The reason says:

Low oxidation states are found when a complex has ligands capable of $\pi$-donor character in addition to the $\sigma$-bonding.

This is incorrect.

In fact, low oxidation states are stabilized by ligands showing $\pi$-acceptor character (also called $\pi$-acid ligands), not $\pi$-donor character.

Examples of such ligands are: $\mathrm{CO},\ \mathrm{NO^+},\ \mathrm{CN^-}$

In $\mathrm{Ni(CO)_4}$ and $\mathrm{Fe(CO)_5}$, CO acts as:

• a $\sigma$-donor through lone pair donation to metal,
• a $\pi$-acceptor through back-bonding from metal to ligand.

This back-bonding helps stabilize the metal in low or zero oxidation state.

Therefore, Reason R is not correct.

Choose the correct option

• Assertion A: Correct
• Reason R: Not correct

Therefore, the correct option is: $\boxed{\text{B}}$

Comparison with stored answer

Stored correct answer = B

My derived answer = B

So, they agree.