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Question #44575

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The Question

Consider the aquation reaction of the optically inactive complex $trans-\ce{[Co(en)2Cl2]+}$ to form $\ce{[Co(en)2Cl(H2O)]^2+}$ at 298 K. The reaction proceeds via a dissociative ( $D$ ) mechanism involving a 5-coordinate intermediate.

Two hypothetical reaction pathways are proposed based on the geometry of the intermediate: Pathway I: The intermediate is a rigid, achiral Square Pyramidal (SP) species where the leaving group's position remains vacant. Pathway II: The SP intermediate completely equilibrates to a Trigonal Bipyramidal (TBP) species (where the remaining $\ce{Cl-}$ ligand occupies an equatorial position) before the attack of $\ce{H2O}$ .

Let $P_I$ be the product(s) formed via Pathway I, and $P_{II}$ be the product(s) formed via Pathway II. Assume that in Pathway II, the attack of $\ce{H2O}$ occurs with equal probability at all sterically accessible equatorial edges of the TBP intermediate.

Which of the following statements correctly describes the stereochemical outcome of $P_I$ and $P_{II}$ ?

Options

A.
$P_I$ is a single achiral compound; $P_{II}$ is a mixture of an achiral compound and a racemic pair of chiral compounds.
B.
$P_I$ is a racemic pair of chiral compounds; $P_{II}$ is a mixture of two different achiral compounds.
C.
$P_I$ is a single achiral compound; $P_{II}$ is a mixture of an achiral compound and a single enantiomer of a chiral compound.
D.
Both $P_I$ and $P_{II}$ are exclusively single achiral compounds, but they are geometric isomers of each other.

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Numerical_Precision

Improve calculation stamina and avoid rounding errors in high-stakes competitive examinations.

Concepts Behind This Question

Geometric Isomerism Requirements|Optical Isomerism Logic|Reaction Progress Coordinate

Why This Question Is Tricky

Common Mistakes

The question tests the stereochemical consequences of reaction mechanisms in coordination chemistry, specifically the difference between rigid and fluxional 5-coordinate intermediates.

A common student trap is the 'retention of configuration bias,' where students assume that an achiral starting material will only yield achiral products.

Furthermore, students often fail to visualize that a TBP intermediate with bidentate ligands spanning axial-equatorial positions is chiral and must form as a racemate, leading to a racemic mixture of chiral cis products alongside the achiral trans product.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Coordination Compounds - Class 12
Concept	Geometric Isomerism Requirements
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free