JEE Advanced 2025 Paper 1 Q02 Chemistry Coordination Chemistry Crystal Field Theory Medium

JEE Advanced 2025 Paper 1 · Q02 · Crystal Field Theory

The correct order of the wavelength maxima of the absorption band in the UV–visible region for the given Co(III) complexes is:

  1. A. $[Co(CN)_6]^{3-} < [Co(NH_3)_6]^{3+} < [Co(NH_3)_5(H_2O)]^{3+} < [Co(NH_3)_5(Cl)]^{2+}$
  2. B. $[Co(NH_3)_5(Cl)]^{2+} < [Co(NH_3)_5(H_2O)]^{3+} < [Co(NH_3)_6]^{3+} < [Co(CN)_6]^{3-}$
  3. C. $[Co(CN)_6]^{3-} < [Co(NH_3)_5(Cl)]^{2+} < [Co(NH_3)_5(H_2O)]^{3+} < [Co(NH_3)_6]^{3+}$
  4. D. $[Co(NH_3)_6]^{3+} < [Co(CN)_6]^{3-} < [Co(NH_3)_5(Cl)]^{2+} < [Co(NH_3)_5(H_2O)]^{3+}$
Reveal answer + step-by-step solution

Correct answer:A

Solution

$\lambda_{\text{max}}$ corresponds to the d–d transition energy: stronger field ligand ⟹ larger $\Delta_o$ ⟹ smaller $\lambda$. Spectrochemical order (relevant ligands): $Cl^- < H_2O < NH_3 < CN^-$. $\Delta_o$ ranking: $[Co(NH_3)_5 Cl]^{2+}$ (smallest, 5 $NH_3$ + 1 $Cl$) $< [Co(NH_3)_5(H_2O)]^{3+} < [Co(NH_3)_6]^{3+} < [Co(CN)_6]^{3-}$ (largest). $\lambda_{\text{max}}$ ranking is the reverse of $\Delta_o$: $[Co(CN)_6]^{3-} < [Co(NH_3)_6]^{3+} < [Co(NH_3)_5(H_2O)]^{3+} < [Co(NH_3)_5(Cl)]^{2+}$. Answer: (A).

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