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Question #44573

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The Question

Glycylglycine (\ce{Gly-Gly}) is a dipeptide that coordinates to transition metals. An octahedral complex $\mathbf{P}$ is synthesized by reacting \ce{Co^{3+}} with two equivalents of \ce{Gly-Gly} in a strongly alkaline aqueous medium (pH > 11). Under these conditions, \ce{Gly-Gly} acts as a fully deprotonated tridentate ligand ( $L^{2-}$ ), coordinating via the terminal amine nitrogen, the amide nitrogen, and the terminal carboxylate oxygen.

Due to the resonance in the deprotonated peptide bond, the ligand is structurally constrained. Determine the total number of stereoisomers of complex $\mathbf{P}$ and the possible bond angle(s) between the two terminal amine nitrogen atoms (\ce{N_{amine}-Co-N_{amine}}) in these isomers.

Options

A.
2 stereoisomers, $90^\circ$ only
B.
4 stereoisomers, $90^\circ$ and $180^\circ$
C.
6 stereoisomers, $90^\circ$ and $180^\circ$
D.
3 stereoisomers, $90^\circ$ only

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Correct Answer

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

Geometric Isomerism Requirements|Peptide Bond Planarity|Amino Acid Zwitterion pH Sensitivity

Why This Question Is Tricky

Common Mistakes

The question tests the intersection of biochemistry and coordination chemistry.

Students often assume tridentate ligands (like dien) can adopt both facial and meridional geometries.

However, the 'Hidden Variable' is the partial double bond character of the peptide bond, which is enhanced upon deprotonation of the amide nitrogen (\ce{-C(O^-)=N-}).

This enforces strict planarity, restricting the ligand to a meridional geometry.

Furthermore, spatial constraints in the octahedral field dictate that the central amide nitrogens must be trans, forcing the terminal amines to be exclusively cis ( $90^\circ$ ), limiting the complex to just one enantiomeric pair (2 stereoisomers).

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