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Question #44554

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The Question

An octahedral complex $X$ is synthesized by reacting aqueous $NiCl_2$ with an excess of ethylenediamine ( $en$ ). Complex $X$ exhibits a violet color. When $X$ is isolated and dissolved in pure liquid ammonia at 298 K (under high pressure), the solution gradually turns blue, indicating the formation of a new octahedral complex $Y$ . Which of the following statements correctly identifies the thermodynamic driving force and the spectroscopic origin of this color change?

Options

A.
The high concentration of solvent $NH_3$ overcomes the chelate effect, driving the inner-sphere displacement of bidentate $en$ by monodentate $NH_3$ . Since $NH_3$ is a weaker field ligand than $en$ , the crystal field splitting energy ( $\Delta_o$ ) decreases, causing a red shift in the d-d absorption band.
B.
$NH_3$ acts as a stronger field ligand than $en$ due to its higher basicity in liquid ammonia, increasing $\Delta_o$ . This causes a blue shift in the d-d absorption band, changing the transmitted color from violet to blue.
C.
The solvent $NH_3$ induces a partial decoordination, converting bidentate $en$ into monodentate $en$ . This reduces the coordination number to 4, forming a tetrahedral complex with a significantly lower $\Delta_t$ , leading to a red shift in absorption.
D.
The liquid ammonia reduces $Ni^{2+}$ to $Ni^+$ , which undergoes a Jahn-Teller distortion. The resulting splitting of the $e_g$ orbitals decreases the effective $\Delta_o$ , causing a red shift in the absorption band.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Ligand Denticity Variations|Color and d-d Transitions|Solutions - Solubility of Solids

Why This Question Is Tricky

Common Mistakes

This question tests the intersection of Le Chatelier's principle, the chelate effect, and Crystal Field Theory.

The 'Chemical Exception' being tested is the assumption that chelate complexes like $[Ni(en)_3]^{2+}$ are completely inert to substitution by monodentate ligands.

Students must recognize that a solvent acting as a ligand (in massive excess) can thermodynamically drive the inner-sphere dissociation of chelate rings.

Furthermore, it tests the ability to correlate transmitted color changes (violet to blue) with absorption shifts (yellow-green to orange), correctly identifying this as a 'red shift' in absorption due to a decrease in $\Delta_o$ .

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Coordination Compounds - Class 12
Concept	Ligand Denticity Variations
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free