Question

Given below are two statements: S-I: \( [\text{CoBr}_4]^{2-} \) absorbs lesser energy than \( [\text{CoCl}_4]^{2-} \) S-II: \( [\text{CoCl}_4]^{2-} \) has higher crystal field splitting energy than \( [\text{CoBr}_4]^{2-} \)

• A. Both S-I and S-II are correct
• B. Both S-I and S-II are incorrect
• C. S-I is correct and S-II is incorrect
• D. S-I is incorrect and S-II is correct

Hint:

Stronger field ligands cause larger crystal field splitting and require absorption of higher energy radiation.

Answer 1

The Correct Option is A

Alternative Method (Orbital Overlap & Metal–Ligand Interaction Approach):

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Crystal field splitting arises due to the repulsion between metal d-orbitals and the electron pairs of ligands approaching the metal ion. The magnitude of splitting depends on how strongly the ligand interacts with the metal center.

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Step 1: Compare metal–ligand interactions

Chloride ions are smaller and less diffuse than bromide ions. Because of this, Cl⁻ can approach the cobalt ion more closely, leading to stronger electrostatic interaction and better orbital overlap.

In contrast, the larger size of Br⁻ results in weaker interaction with the metal ion.

Δ(Cl⁻) > Δ(Br⁻)

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Step 2: Evaluate Statement S-I

The complex [CoBr₄]²⁻ contains the weaker ligand Br⁻. Weaker metal–ligand interaction leads to smaller crystal field splitting.

Since the energy absorbed during a d–d transition is directly proportional to Δ, this complex absorbs less energy.

Hence, Statement S-I is correct.

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Step 3: Evaluate Statement S-II

The complex [CoCl₄]²⁻ contains the stronger ligand Cl⁻. Stronger interaction results in larger crystal field splitting energy.

Therefore:

Δ[CoCl₄]²⁻ > Δ[CoBr₄]²⁻

Hence, Statement S-II is correct.

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Final Conclusion:

Both Statement S-I and Statement S-II are correct.