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Question #44113

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The Question

Consider the stepwise formation of the complex $[Fe(phen)_3]^{2+}$ from $[Fe(H_2O)_6]^{2+}$ (aq) and 1,10-phenanthroline ( $phen$ ) at 298 K. The stepwise formation constants are denoted as $K_1$ , $K_2$ , and $K_3$ . Which of the following correctly represents the trend in these equilibrium constants and identifies the primary electronic/thermodynamic driving force for this specific trend?

Options

A.
$K_1 > K_2 \ll K_3$ ; The third substitution triggers a high-spin to low-spin crossover ( $t_{2g}^4 e_g^2 \rightarrow t_{2g}^6$ ), maximizing CFSE and synergic \pi-backbonding into the $phen$ LUMO.
B.
$K_1 < K_2 < K_3$ ; The chelate effect provides a strictly increasing entropic driving force (\Delta S^\circ > 0) for each successive bidentate ligand addition.
C.
$K_1 > K_2 > K_3$ ; Successive substitutions are statistically less probable and sterically hindered, which dominates over the strong-field nature of the ligand.
D.
$K_1 < K_2 > K_3$ ; The second ligand completes the equatorial plane, maximizing \sigma-donation, while the third ligand suffers from severe steric repulsion.

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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

Successive Equilibrium Constants|Synergic Bonding in Carbonyls

Why This Question Is Tricky

Common Mistakes

Students often memorize that stepwise formation constants strictly decrease ( $K_1 > K_2 > K_3$ ) due to statistical and steric factors, or they mistakenly assume the 'Chelate Effect' causes a continuous increase ( $K_1 < K_2 < K_3$ ).

This question tests the 'Chemical Exception' where an abrupt change in the electronic ground state (high-spin to low-spin crossover) driven by cumulative ligand field strength and synergic \pi-backbonding causes a massive, anomalous spike in the final formation constant ( $K_3 \gg K_2$ ).

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Equilibrium - Class 11 Part 1
Concept	Successive Equilibrium Constants
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free