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JEE MAIN - Chemistry (2023 - 11th April Evening Shift - No. 4)

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

Assertion A : $$\left[\mathrm{CoCl}\left(\mathrm{NH}_{3}\right)_{5}\right]^{2+}$$ absorbs at lower wavelength of light with respect to $$\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5}\left(\mathrm{H}_{2} \mathrm{O}\right)\right]^{3+}$$

Reason R : It is because the wavelength of the light absorbed depends on the oxidation state of the metal ion.

In the light of the above statements, choose the correct answer from the options given below:

$$\mathrm{A}$$ is false but $$\mathrm{R}$$ is true
Both $$\mathrm{A}$$ and $$\mathrm{R}$$ are true but $$\mathrm{R}$$ is NOT the correct explanation of $$A$$
Both $$\mathrm{A}$$ and $$\mathrm{R}$$ are true and $$\mathrm{R}$$ is the correct explanation of $$\mathrm{A}$$
$$\mathrm{A}$$ is true but $$\mathrm{R}$$ is false

Explanation

Assertion A : $$\left[\mathrm{CoCl}\left(\mathrm{NH}_{3}\right)_{5}\right]^{2+}$$ absorbs at a lower wavelength of light with respect to $$\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5}\left(\mathrm{H}_{2} \mathrm{O}\right)\right]^{3+}$$.

Given that H2O is a stronger field ligand than Cl-, it would result in a larger splitting of the d-orbitals and hence a larger energy difference. This would lead to absorption at a lower wavelength. Therefore, the Assertion A is false.

Reason R : It is because the wavelength of the light absorbed depends on the oxidation state of the metal ion.

While it's true that the oxidation state can influence the absorption wavelength by affecting the energy levels of the d-orbitals, the difference in the absorption wavelength in this specific case is not primarily due to the oxidation state (the oxidation state of Co in both complexes is the same (+3)), but rather due to the difference in ligand field strength (H2O vs Cl-). Therefore, the Reason R is not providing the correct explanation for Assertion A.

So, the correct answer would be:

Option A : A is false but R is true.

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