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JEE MAIN - Chemistry (2023 - 6th April Evening Shift - No. 7)

Given below are two statements: one is labelled as "Assertion A" and the other is labelled as "Reason R"

Assertion A : In the complex $$\mathrm{Ni}(\mathrm{CO})_{4}$$ and $$\mathrm{Fe}(\mathrm{CO})_{5}$$, the metals have zero oxidation state.

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

In the light of the above statements, choose the most appropriate answer from the options given below

A is not correct but R is correct
A is correct but R is not correct
Both A and R are correct but R is NOT the correct explanation of A
Both A and R are correct and R is the correct explanation of A

Explanation

  • Option B: "A is correct but R is incorrect" is the right answer. Here is the reasoning:

    • Assertion A is correct. In the complex $$\mathrm{Ni(CO)_4}$$, if we assume the oxidation state of Ni to be x, we can set up the equation:

    $ x + (0 \times 4) = 0 $

    From which we find that (x = 0).

    Similarly, in $$\mathrm{Fe(CO)_5}$$, if we let the oxidation state of Fe be y, the equation becomes:

    $ y + (0 \times 5) = 0 $

    Solving this gives (y = 0).

    Therefore, the oxidation states of both Ni and Fe are zero in their respective complexes, verifying that Assertion A is true.

    Regarding Reason R, it is incorrect. The low oxidation states in these complexes are stabilized through synergic bonding, which involves the ligand being a $$\pi$$-acceptor, not a $$\pi$$-donor. The CO ligand in these complexes accepts electron density from the metal into its antibonding molecular orbitals, characterizing a $$\pi$$-acid ligand, and not a $$\pi$$-donor ligand as stated. Consequently, Reason R does not hold true.

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