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Question #44577

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The Question

Consider the ligand substitution reaction of the enantiomerically pure octahedral complex $\Lambda$ - $\text{cis}$ - $\ce{[Co(phen)2Cl2]+}$ with $\ce{NaN3}$ in an aqueous solution at 298 K. A student applies organic $S_N2$ heuristics, predicting a purely associative pathway leading to 100% inversion of configuration (yielding the $\Delta$ - $\text{cis}$ isomer). However, the experimentally isolated product is a racemic mixture of $\Lambda$ - and $\Delta$ - $\text{cis}$ - $\ce{[Co(phen)2(Cl)(N3)]+}$ . Which of the following best explains this stereochemical outcome?

Options

A.
The steric bulk of the ancillary phenanthroline ligands blocks the 7-coordinate associative pathway, forcing a dissociative mechanism. The resulting 5-coordinate trigonal bipyramidal intermediate permits pseudorotation, leading to racemization.
B.
The reaction proceeds via an associative ( $S_N2$ -like) mechanism, but the strong trans-effect of the chloride ligand directs the incoming azide to the trans position, yielding an achiral meso complex.
C.
The complex undergoes an $S_N1CB$ mechanism where the phenanthroline ligand is deprotonated, forming a planar intermediate that allows equal probability of attack from both faces.
D.
The azide ligand acts as a bidentate chelator in the transition state, forcing a temporary reduction of $\ce{Co^{III}}$ to $\ce{Co^{II}}$ , which rapidly racemizes due to its high lability before re-oxidizing.

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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

Optical Isomerism Logic|SN2 Stereochemical Inversion|Steric Hindrance in SN2

Why This Question Is Tricky

Common Mistakes

This question tests the 'Chemical Exception' where students erroneously apply organic $S_N2$ stereochemical heuristics (Walden inversion) to inorganic octahedral complexes.

The hidden variable is the steric bulk of the ancillary ligands (phenanthroline), which strictly forbids the 7-coordinate associative transition state required for an $S_N2$ -like mechanism in octahedral complexes.

Instead, it forces a dissociative pathway via a 5-coordinate trigonal bipyramidal intermediate.

This intermediate undergoes pseudorotation, leading to racemization.

Furthermore, it traps students who might guess the $S_N1CB$ mechanism (Option C), as phenanthroline lacks the acidic N-H protons required for conjugate base formation.

Quick Facts About This Question

Question Information

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