Question

The correct order of wavelength of absorption ($\lambda_{\max}$) of the Cr-complexes is (en = ethylenediamine)

• A. [CrF$_6$]$^{3-}$>[Cr(H$_2$O)$_6$]$^{3+}$>[Cr(en)$_3$]$^{3+}$>[Cr(CN)$_6$]$^{3-}$
• B. [Cr(H$_2$O)$_6$]$^{3+}$>[CrF$_6$]$^{3-}$>[Cr(en)$_3$]$^{3+}$>[Cr(CN)$_6$]$^{3-}$
• C. [Cr(CN)$_6$]$^{3-}$>[Cr(en)$_3$]$^{3+}$>[Cr(H$_2$O)$_6$]$^{3+}$>[CrF$_6$]$^{3-}$
• D. [Cr(en)$_3$]$^{3+}$>[Cr(CN)$_6$]$^{3-}$>[Cr(H$_2$O)$_6$]$^{3+}$>[CrF$_6$]$^{3-}$

Hint:

Weaker ligands give longer wavelength; stronger ligands give shorter wavelength absorption.

Answer 1

The Correct Option is A

Step 1: Relation between $\lambda_{\max$ and ligand field strength.}
Higher ligand field splitting (Δ) → higher energy absorption → lower wavelength. Lower ligand field splitting → higher wavelength.
Step 2: Ligand field strength order.
From weak field to strong field:
F$^-$<H$_2$O<en<CN$^-$.
Step 3: Wavelength trend.
Weaker ligand → smaller Δ → absorbs longer wavelength.
Thus: [Cr(H$_2$O)$_6$]$^{3+}$ (weak) → highest λ
[CrF$_6$]$^{3-}$ → slightly lower λ
[Cr(en)$_3$]$^{3+}$ → lower λ
[Cr(CN)$_6$]$^{3-}$ (strongest field) → lowest λ
Step 4: Conclusion.
Correct order is option (B).