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Question #44905

Difficulty: 8/10 | 🟢 Free

The Question

Consider the standard reduction potentials ( $E^\circ$ ) at 298 K for the $\ce{Fe^{3+}/Fe^{2+}}$ redox couple in three different octahedral coordination environments:

(I) $\ce{[Fe(H2O)6]^{3+} + e^- \rightleftharpoons [Fe(H2O)6]^{2+}}$ (II) $\ce{[Fe(CN)6]^{3-} + e^- \rightleftharpoons [Fe(CN)6]^{4-}}$ (III) $\ce{[Fe(phen)3]^{3+} + e^- \rightleftharpoons [Fe(phen)3]^{2+}}$ (where phen = 1,10-phenanthroline)

A mild reducing agent is introduced to an equimolar mixture of the three oxidized complexes. It selectively reduces only the complex with the highest $E^\circ$ value. Let the reduced product of this reaction be Complex $X$ .

Identify the correct decreasing order of $E^\circ$ values for the three couples. Furthermore, according to Werner's Coordination Theory, determine the spin-only magnetic moment ( $\mu_s$ ) and the ratio of Primary Valency to Secondary Valency ( $PV/SV$ ) for the central metal in Complex $X$ .

Options

A.
$III > I > II$ ; $\mu_s = 0$ B.M. ; $PV/SV = 1/3$
B.
$II > III > I$ ; $\mu_s = 0$ B.M. ; $PV/SV = 1/2$
C.
$III > II > I$ ; $\mu_s = 4.90$ B.M. ; $PV/SV = 1/2$
D.
$I > III > II$ ; $\mu_s = 4.90$ B.M. ; $PV/SV = 1/3$

Check Your Answer

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

Werner’s Theory Postulates|E° (M³⁺/M²⁺) Redox Potential

Why This Question Is Tricky

Common Mistakes

This question tests the 'Hidden Variable' of electrostatic repulsion versus Crystal Field Stabilization Energy (CFSE) in determining redox potentials.

Students often fall into the trap of assuming that stronger ligands (like $\ce{CN^-}$ ) always increase the $E^\circ$ of $\ce{M^{3+}/M^{2+}}$ couples by stabilizing the $d^6$ state (predicting $II > III > I$ ).

However, they overlook the electrostatic penalty of forming a highly charged $\ce{[Fe(CN)6]^{4-}}$ anion.

Furthermore, the problem integrates Werner's Theory by requiring the calculation of the Primary to Secondary Valency ratio for the dynamically formed reduced state, trapping students who calculate it for the oxidized state ( $3/6 = 1/2$ ).

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Coordination Compounds - Class 12
Concept	Werner’s Theory Postulates
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free