Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: [CoCl(NH₃)]^2– absorbs at lower wavelength of light with respect to [Co(NH₃)₅(H₂O) ^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:

• A. Both A and R are true and R is the correct explanation of A
• B. A is true but R is false
• C. Both A and R are true but R is NOT the correct explanation of A
• D. A is false but R is true

Answer 1

The Correct Option is D

Assertion (A):

The assertion states that \([ \text{CoCl}(\text{NH}_3)_5 ]^{2+}\) absorbs at a lower wavelength compared to \([ \text{CoCl}(\text{NH}_3)_5(\text{H}_2\text{O}) ]^{3+}\).

• This assertion is false. The complex \([ \text{CoCl}(\text{NH}_3)_5(\text{H}_2\text{O}) ]^{3+}\) contains \( \text{Co}^{3+} \), which has a larger crystal field splitting energy (CFSE) than \( \text{Co}^{2+} \) in \([ \text{CoCl}(\text{NH}_3)_5 ]^{2+}\).
• Additionally, \( \text{H}_2\text{O} \) is a stronger field ligand than \( \text{Cl}^- \), which further increases the CFSE for \([ \text{CoCl}(\text{NH}_3)_5(\text{H}_2\text{O}) ]^{3+} \).
• A larger CFSE corresponds to absorption of higher energy (shorter wavelength) light.
• Thus, \([ \text{CoCl}(\text{NH}_3)_5(\text{H}_2\text{O}) ]^{3+}\) should absorb at a lower wavelength compared to \([ \text{CoCl}(\text{NH}_3)_5 ]^{2+}\).

Reason (R):

The reason states that the wavelength of light absorbed depends on the oxidation state of the metal ion. This is true. A higher oxidation state of the central metal ion increases the CFSE, resulting in the absorption of light at a lower wavelength (higher energy).

Final Answer:

Assertion A is false, but Reason R is true.