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Question #44309

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The Question

Consider the following substitution reactions of $Co(III)$ complexes in aqueous solution at 298 K:

Reaction I: $[Co(NH_3)_5(H_2O)]^{3+} + SCN^- \rightleftharpoons P + H_2O$

Reaction II: $[Co(CN)_5(H_2O)]^{2-} + SCN^- \rightleftharpoons Q + H_2O$

According to the Hard-Soft Acid-Base (HSAB) principle and Jørgensen's concept of symbiosis, the spectator ligands alter the effective electronegativity, polarizability, and absolute hardness ( $\eta$ ) of the central metal electrophile. Identify the coordinating atom of the ambidentate thiocyanate ligand in the major products $P$ and $Q$ , respectively, and the correct relationship between the absolute hardness of the metal center in the reactive intermediates of I ( $\eta_I$ ) and II ( $\eta_{II}$ ).

Options

A.
P: N-bonded, Q: S-bonded, $\eta_I > \eta_{II}$
B.
P: S-bonded, Q: N-bonded, $\eta_I < \eta_{II}$
C.
P: N-bonded, Q: N-bonded, $\eta_I > \eta_{II}$
D.
P: S-bonded, Q: S-bonded, $\eta_I < \eta_{II}$

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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

Electronegativity Scales|Ambidentate Linkage Isomerism|Electrophile vs Nucleophile

Why This Question Is Tricky

Common Mistakes

This question tests the concept of 'Symbiosis' in HSAB theory, where the hardness/softness (and effective electronegativity) of a metal electrophile is not static but dynamically altered by its spectator ligands.

Students often rote-memorize that $Co^{3+}$ is a hard acid and will always bind to the N-terminus of $SCN^-$ , failing to account for the softening effect of the highly polarizable $CN^-$ ligands which shifts the thermodynamic preference to the S-terminus.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Classification of Elements and Periodicity in Properties - Class 11 Part 1
Concept	Electronegativity Scales
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free