JEE Advanced 2025 — Paper 1 — CHEM-I solution

JEE Advanced2025Paper 1CHEM-I

The correct order of the wavelength maxima of the absorption band in the ultraviolet-visible region for the given complexes is

A
[Co(CN) $_{6}$ ] $^{3 -}$ < [Co(NH $_{3}$ ) $_{6}$ ] $^{3 +}$ < [Co(NH $_{3}$ ) $_{5}$ (H $_{2}$ O)] $^{3 +}$ < [Co(NH $_{3}$ ) $_{5}$ (Cl)] $^{2 +}$
B
[Co(NH $_{3}$ ) $_{5}$ (Cl)] $^{2 +}$ < [Co(NH $_{3}$ ) $_{5}$ (H $_{2}$ O)] $^{3 +}$ < [Co(NH $_{3}$ ) $_{6}$ ] $^{3 +}$ < [Co(CN) $_{6}$ ] $^{3 -}$
C
[Co(CN) $_{6}$ ] $^{3 -}$ < [Co(NH $_{3}$ ) $_{5}$ (Cl)] $^{2 +}$ < [Co(NH $_{3}$ ) $_{5}$ (H $_{2}$ O)] $^{3 +}$ < [Co(NH $_{3}$ ) $_{6}$ ] $^{3 +}$
D
[Co(NH $_{3}$ ) $_{6}$ ] $^{3 +}$ < [Co(CN) $_{6}$ ] $^{3 -}$ < [Co(NH $_{3}$ ) $_{5}$ (Cl)] $^{2 +}$ < [Co(NH $_{3}$ ) $_{5}$ (H $_{2}$ O)] $^{3 +}$

Solution

The wavelength of maximum absorption is inversely related to the crystal field splitting energy: $λ_{m a x} \propto \frac{1}{Δ _{O}}$ .

From the spectrochemical series for ligand field strength: CN $^{-}$ > NH $_{3}$ > H $_{2}$ O > Cl $^{-}$ . Hence $Δ_{O}$ decreases in the same order:

[Co(CN) $_{6}$ ] $^{3 -}$ > [Co(NH $_{3}$ ) $_{6}$ ] $^{3 +}$ > [Co(NH $_{3}$ ) $_{5}$ (H $_{2}$ O)] $^{3 +}$ > [Co(NH $_{3}$ ) $_{5}$ (Cl)] $^{2 +}$

Inverting for wavelength gives the order in option (A).

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