Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A : [CoCl(NH₃)₅]²⁺ absorbs at lower wavelength of light with respect to [Co(NH₃)₅(H₂ O)]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:

Solution

Chemistry ma’am · 4 steps

Both complexes are Co(III), so the only difference is the sixth ligand: Cl⁻ against H₂O.

1. Oxidation state. In [CoCl(NH₃)₅]²⁺, x - 1 = +2 gives x = +3. In [Co(NH₃)₅(H₂O)]³⁺, both ligands are neutral, so x = +3. Both complexes contain Co³⁺.
2. Compare the ligands. Five NH₃ ligands are common to both complexes. In the spectrochemical series Cl⁻ < H₂O < NH₃, so Cl⁻ is the weaker field ligand and gives a smaller splitting Δₒ.

   Complex	Co oxidation state	Sixth ligand	Δₒ	Wavelength absorbed
   [CoCl(NH₃)₅]²⁺	+3	Cl⁻ (weaker field)	smaller	longer
   [Co(NH₃)₅(H₂O)]³⁺	+3	H₂O (stronger field)	larger	shorter

3. Check A and R. A smaller Δₒ means lower energy and so a longer wavelength, since E = hc/λ. The chloro complex absorbs at a higher wavelength, so A is false. R is a true general fact: a higher oxidation state gives a larger Δₒ. It just does not apply here, because both metals are +3.
4. So. A is false and R is true, which is option (a).

The idea to keep. Compare oxidation states first. If they are equal, the ligands decide. A weaker field ligand gives a smaller Δₒ, so the complex absorbs lower energy light at a longer wavelength.

Answer from the NTA final answer key. Solution by PrepWiser.