Question

Three complexes,

[CoCl(NH₃)₅]²⁺ (I),

[Co(NH₃)₅H₂O]³⁺ (II) and 

[Co(NH₃)₆]³⁺ (III) 

absorb light in the visible region. The correct order of the wavelength of light absorbed by them is:

Options

• (III) > (I) > (II)

• (III) > (II) > (I)

• (I) > (II) > (III)

• (II) > (I) > (III)

Answer 1

(I) > (II) > (III)

Explanation:

• [CoCl(NH₃)₅]²⁺ (I) has chloride (Cl⁻), a weak field ligand, resulting in the smallest splitting and absorption at the longest wavelength (lowest energy).
• [Co(NH₃)₅H₂O]³⁺ (II) has ammonia and water, causing moderate splitting and absorption at a medium wavelength.
• [Co(NH₃)₆]³⁺ (III) has ammonia (NH₃), a strong field ligand, leading to the largest splitting and absorption at the shortest wavelength (highest energy).