Question

Consider the metal complexes \( [Ni(en)_3]^{2+} \) (A), \( [NiCl_4]^{2-} \) (B) and \( [Ni(NH_3)_6]^{2+} \) (C). Choose the CORRECT option by considering the number of unpaired electrons present in (A), (B) and (C) respectively and the order of frequency of absorption.

• A. 2, 2, 2 and (A) > (C) > (B)
• B. 0, 2, 0 and (A) > (C) > (B)
• C. 2, 2, 0 and (B) > (C) > (A)
• D. 2, 2, 2 and (C) > (A) > (B)

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
All complexes contain \( Ni^{2+} \), which has a \( d^8 \) configuration. The number of unpaired electrons depends on geometry and ligand field strength. The frequency of absorption (\( \nu \)) is directly proportional to the crystal field splitting energy (\( \Delta \)), which follows the spectrochemical series.

Step 2: Key Formula or Approach:
1. \( Ni^{2+} \) octahedral (\( d^8 \)): Always 2 unpaired electrons ( \( t_{2g}^6 e_g^2 \) ).
2. \( Ni^{2+} \) tetrahedral: Always 2 unpaired electrons ( \( e^4 t_2^4 \) ).
3. Spectrochemical Series: \( en>NH_3>Cl^- \).

Step 3: Detailed Explanation:
- (A) \( [Ni(en)_3]^{2+} \): Octahedral, \( d^8 \). Unpaired \( e^- = 2 \). Ligand 'en' is strong field.
- (B) \( [NiCl_4]^{2-} \): Tetrahedral, \( d^8 \). Unpaired \( e^- = 2 \). Ligand \( Cl^- \) is weak field.
- (C) \( [Ni(NH_3)_6]^{2+} \): Octahedral, \( d^8 \). Unpaired \( e^- = 2 \). Ligand \( NH_3 \) is moderate field.
Frequency order: \( \Delta \) is proportional to ligand strength. Also, \( \Delta_{oct}>\Delta_{tet} \).
Ligand strength: \( en>NH_3>Cl^- \).
Thus, \(\Delta_A>\Delta_C>\Delta_B \).
Frequency order: (A) > (C) > (B).

Step 4: Final Answer:
The complexes each have 2 unpaired electrons, and the absorption frequency order is A > C > B.