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Question #44899

Difficulty: 8/10 | 🟢 Free

The Question

Transition metals can achieve unusually high oxidation states when coordinated to highly electronegative ligands such as $O^{2-}$ and $F^-$ . Consider two such high oxidation state complexes: Potassium ferrate, \ce{K2FeO4} (Complex P), and Potassium hexafluoridonickelate(IV), \ce{K2[NiF6]} (Complex Q). Assuming ideal spin-only behavior, what are the respective magnetic moments ( $\mu_{eff}$ ) of Complex P and Complex Q at 298 K?

Options

A.
2.83 BM and 0 BM
B.
0 BM and 4.90 BM
C.
2.83 BM and 4.90 BM
D.
0 BM and 0 BM

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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

Stability of High O.S. Fluorides/Oxides|Spin-Only Magnetic Moment

Why This Question Is Tricky

Common Mistakes

This question tests two major exceptions in coordination chemistry.

First, it challenges the cognitive bias that maximum group oxidation states or very high oxidation states (like +6) always result in a $d^0$ diamagnetic configuration (e.g., \ce{CrO4^{2-}}, \ce{MnO4^-}). \ce{Fe^{6+}} is $d^2$ and paramagnetic.

Second, it tests the hidden variable in Crystal Field Theory: the effect of the metal's oxidation state on $\Delta_o$ .

Students typically memorize $F^-$ as a weak-field ligand and would incorrectly assign a high-spin $d^6$ configuration ( $n=4$ , $\mu=4.90$ BM) to \ce{[NiF6]^{2-}}.

However, a +4 oxidation state on a 3d metal increases $\Delta_o$ so much that it forces a low-spin, diamagnetic configuration regardless of the ligand's position in the spectrochemical series.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	D and F Block Elements - Class 12
Concept	Stability of High O.S. Fluorides/Oxides
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free