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Question #44593

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The Question

An optically active, violet-colored coordination compound $\textbf{X}$ has the empirical formula $\ce{CoCl3 \cdot 2en}$ (where $\ce{en}$ = ethylenediamine). When a dilute aqueous solution of $\textbf{X}$ is maintained at $298\text{ K}$ , it undergoes a first-order reaction to form a new complex $\textbf{Y}$ . During this process, the solution gradually loses its optical activity and its color changes from violet to pinkish-red. Which of the following statements correctly identifies the mechanism responsible for the loss of optical activity and the fundamental cause of the color change?

Options

A.
Dissociation of a $\ce{Cl^-}$ ligand forms a trigonal bipyramidal intermediate leading to stereochemical scrambling, while substitution by $\ce{H2O}$ increases $\Delta_o$ , shifting the absorption maximum to a shorter wavelength.
B.
Spontaneous reduction of $\ce{Co^{3+}}$ to $\ce{Co^{2+}}$ creates a labile $d^7$ complex that rapidly racemizes, and the change in oxidation state alters the d-d transition energy.
C.
Association of $\ce{H2O}$ forms a chiral pentagonal bipyramidal intermediate that undergoes reductive elimination, decreasing $\Delta_o$ and shifting the absorption maximum to a longer wavelength.
D.
Isomerization to the thermodynamically stable \textit{trans}-isomer occurs without ligand exchange, and the color change is driven by a transition from a low-spin to a high-spin state.

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Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

Color and d-d Transitions|Optical Isomerism Logic|Valence Bond Theory Hybridization

Why This Question Is Tricky

Common Mistakes

This problem tests the intersection of coordination kinetics, stereochemistry, and Crystal Field Theory.

Students often rely on the heuristic that a dramatic color change coupled with a loss of optical activity implies a redox reaction (e.g., reduction to a labile $\ce{Co^{2+}}$ species).

The 'hidden mechanism' here is the dissociative aquation pathway: the formation of a trigonal bipyramidal intermediate explains the stereochemical scrambling (loss of chirality), while the spectrochemical series ( $\ce{H2O} > \ce{Cl^-}$ ) explains the increase in $\Delta_o$ and the subsequent blue-shift in absorption (resulting in a red-shift in transmitted color), entirely without a change in oxidation state.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Coordination Compounds - Class 12
Concept	Color and d-d Transitions
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free