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Question #44873

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The Question

Consider the species $\ce{Ce^{3+}}$ and $\ce{Ta^{4+}}$ in their respective octahedral complexes at $298\text{ K}$ . Both ions possess a single unpaired electron outside a closed-shell core. While the Lanthanoid contraction significantly affects the atomic radii of 5d transition metals, their magnetic behavior diverges fundamentally from that of the Lanthanoids. Which of the following options correctly provides the theoretical magnetic moment ( $\mu_{eff}$ ) of $\ce{Ce^{3+}}$ (assuming only the ground state multiplet is populated) and the primary reason for the magnetic behavior of $\ce{[TaCl_6]^{2-}}$ ?

Options

A.
$\mu_{eff}(\ce{Ce^{3+}}) \approx 2.54\text{ BM}$ ; In $\ce{[TaCl_6]^{2-}}$ , the free-ion orbital angular momentum is largely quenched by a strong crystal field due to the large spatial extent of the 5d orbitals.
B.
$\mu_{eff}(\ce{Ce^{3+}}) \approx 1.73\text{ BM}$ ; In $\ce{[TaCl_6]^{2-}}$ , the orbital angular momentum remains unquenched because the Lanthanoid contraction effectively shields the 5d orbitals from the ligand field.
C.
$\mu_{eff}(\ce{Ce^{3+}}) \approx 2.54\text{ BM}$ ; In $\ce{[TaCl_6]^{2-}}$ , the orbital angular momentum remains unquenched because massive spin-orbit coupling in heavy 5d metals completely dominates the crystal field splitting.
D.
$\mu_{eff}(\ce{Ce^{3+}}) \approx 1.73\text{ BM}$ ; Both complexes exhibit complete quenching of orbital angular momentum due to their high effective nuclear charges contracting the valence orbitals.

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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

Magnetic Moment Quenching|Post-Lanthanoid Radii Equality|D and F Block Elements

Why This Question Is Tricky

Common Mistakes

This question tests a critical exception and conceptual trap regarding the Lanthanoid Contraction.

Students often erroneously assume that because the Lanthanoid contraction causes 5d metals (like Ta) to have similar radii to their 4d congeners, their valence electrons are similarly 'contracted' and shielded like the 4f electrons in Lanthanoids.

The trap is conflating physical similarity (radii) with electronic/magnetic similarity.

In reality, 4f electrons are buried and exhibit unquenched orbital momentum (requiring the Landé g-factor calculation, not the spin-only formula), whereas 5d electrons are highly extended valence orbitals that experience massive crystal field splitting, which quenches the free-ion orbital angular momentum.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	D and F Block Elements - Class 12
Concept	Magnetic Moment Quenching
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free