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Question #44265

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The Question

A 3d-block metal $M$ forms an octahedral aqua complex $[M(H_2O)_6]^{3+}$ which exhibits an extremely weak d-d transition spectrum because all such transitions are both Laporte and spin-forbidden. The metal $M$ forms a sulfate salt $MSO_4$ . When an aqueous solution of $MSO_4$ is treated with an excess of $KCN$ , a stable octahedral complex $Y$ is formed. Assume a hypothetical scenario where the ionization of $M$ to $M^{2+}$ incorrectly occurs by removing electrons from the $3d$ subshell before the $4s$ subshell. Based on this hypothetical electronic configuration, what would be the predicted spin-only magnetic moment ( $\mu_{hypo}$ ) of complex $Y$ ? Also, determine the true Crystal Field Stabilization Energy (CFSE) of complex $Y$ (ignoring pairing energy $P$ ).

Options

A.
$\mu_{hypo} = 2.83 \text{ BM}$ , True CFSE = $-2.4 \Delta_o$
B.
$\mu_{hypo} = 4.90 \text{ BM}$ , True CFSE = $-0.4 \Delta_o$
C.
$\mu_{hypo} = 0.00 \text{ BM}$ , True CFSE = $-2.4 \Delta_o$
D.
$\mu_{hypo} = 3.87 \text{ BM}$ , True CFSE = $-2.0 \Delta_o$

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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

Pauli Exclusion Principle|d-block Variable Oxidation States|Color and d-d Transitions

Why This Question Is Tricky

Common Mistakes

This problem tests the energy crossover between 4s and 3d orbitals during ionization.

Students often memorize the Aufbau principle (4s fills before 3d) but fail to realize that upon ionization, 4s electrons are removed first because the 3d orbitals drop below 4s in energy once occupied.

The 'trap' forces students to apply Crystal Field Theory (CFT) to a hypothetical anomalous electronic configuration ( $4s^2 3d^4$ ).

They must recognize that even in this hypothetical state, the strong field ligand $CN^-$ forces the $3d^4$ electrons to pair up in the $t_{2g}$ level, yielding $n=2$ instead of $n=4$ .

Additionally, it requires identifying Iron via the spectroscopic hallmark of a completely spin-forbidden d-d spectrum in a high-spin $d^5$ system.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Structure of Atom - Class 11 Part 1
Concept	Pauli Exclusion Principle
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free