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Question #44146

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The Question

In an aqueous acidic medium at $298\text{ K}$ , the $\ce{Mn^{3+}_{(aq)}}$ ion is thermodynamically unstable and undergoes spontaneous disproportionation into $\ce{Mn^{2+}_{(aq)}}$ and $\ce{MnO2_{(s)}}$ . However, when excess ethylenediamine ( $en$ ) is added to the solution, the disproportionation reaction is completely suppressed, yielding a stable octahedral complex $\ce{[Mn(en)3]^{3+}}$ .

Which of the following statements correctly identifies the primary thermodynamic reason for the suppression of disproportionation, and what is the spin-only magnetic moment ( $\mu$ ) of the resulting complex?

Options

A.
The strong-field $en$ ligands induce a low-spin $t_{2g}^4$ configuration, providing a large Crystal Field Stabilization Energy (CFSE) that lowers the $E^\circ_{\ce{Mn^{3+}/Mn^{2+}}}$ reduction potential, making disproportionation thermodynamically unfavorable; $\mu = 2.83\text{ BM}$ .
B.
The bulky chelating $en$ ligands create severe steric hindrance around the metal center, kinetically blocking the bimolecular collision required for the disproportionation mechanism; $\mu = 4.90\text{ BM}$ .
C.
The $en$ ligands stabilize the $+3$ oxidation state by maintaining a high-spin $t_{2g}^3 e_g^1$ configuration with maximum exchange energy, which increases the $E^\circ_{\ce{Mn^{4+}/Mn^{3+}}}$ reduction potential; $\mu = 4.90\text{ BM}$ .
D.
The chelate effect selectively stabilizes the $d^5$ configuration of the $+2$ state, increasing the $E^\circ_{\ce{Mn^{3+}/Mn^{2+}}}$ reduction potential and driving the equilibrium towards the reactants; $\mu = 2.83\text{ BM}$ .

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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

Disproportionation Constraints|Ligand Denticity Variations|Crystal Field Splitting (Octahedral)

Why This Question Is Tricky

Common Mistakes

This problem tests the intersection of Coordination Chemistry and Electrochemistry.

The 'Hidden Variable' is the dependence of standard reduction potentials on the ligand field (CFSE).

Students often treat redox potentials as intrinsic, immutable properties of the metal ion, forgetting that $E^\circ$ values are dictated by the relative stabilities of the complexes involved.

The trap lies in assuming that a strong chelating agent merely provides a kinetic barrier (steric hindrance) or failing to recognize the high-spin to low-spin transition ( $d^4$ high-spin $\mu=4.90\text{ BM}$ to low-spin $\mu=2.83\text{ BM}$ ) that provides the thermodynamic driving force to alter the redox potential.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Redox Reactions - Class 11 Part 2
Concept	Disproportionation Constraints
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free