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Question #45101

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The Question

An aqueous solution of $\ce{FeSO4}$ is treated separately with an excess of 1,10-phenanthroline ( $L_A$ ) and 2,9-dimethyl-1,10-phenanthroline ( $L_B$ ) at 298 K. Let the resulting major iron-containing complex species be $C_A$ and $C_B$ , respectively. Which of the following statements correctly describes the thermodynamic stability and magnetic properties of these complexes?

Options

A.
$C_A$ is a diamagnetic, low-spin octahedral complex, whereas $C_B$ is paramagnetic because the steric bulk of $L_B$ prevents the close approach required to induce a strong octahedral crystal field.
B.
Both $C_A$ and $C_B$ are diamagnetic, low-spin complexes, but $C_B$ has a higher overall formation constant ( $\beta_3$ ) due to the electron-donating $+I$ effect of the methyl groups.
C.
$C_A$ is paramagnetic with a $t_{2g}^4 e_g^2$ configuration, while $C_B$ is diamagnetic because the methyl groups increase the basicity and $\sigma$ -donor strength of $L_B$ .
D.
Both $C_A$ and $C_B$ obey the Effective Atomic Number (EAN) rule, but $C_B$ exhibits a larger Crystal Field Splitting Energy ( $\Delta_o$ ) due to the restricted conformational mobility of $L_B$ .

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Explanation

How the Examiner Thinks

Mechanism_Trap

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Concepts Behind This Question

Steric Hindrance (B-Strain)|Crystal Field Splitting (Octahedral)|Geometric Isomerism Requirements

Why This Question Is Tricky

Common Mistakes

The question tests the interplay between electronic effects and steric hindrance in Coordination Chemistry.

The 'Hidden Variable' is the steric bulk (B-strain) introduced by the methyl groups at the 2,9-positions of 1,10-phenanthroline.

Students often rely solely on the spectrochemical series and inductive effects (+I of methyl groups), erroneously concluding that 2,9-dimethyl-1,10-phenanthroline is a stronger field ligand than 1,10-phenanthroline.

Failing to account for the spatial requirements of bulky ligands leads to the incorrect prediction of a highly stable, diamagnetic octahedral complex for $C_B$ .

In reality, the steric clash prevents close metal-ligand approach, drastically lowering the Crystal Field Splitting Energy (CFSE) and resulting in a weak-field, paramagnetic complex.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Organic Chemistry – Some Basic Principles and Techniques - Class 11 Part 2
Concept	Steric Hindrance (B-Strain)
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free