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Question #44856

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The Question

A black mineral $\textbf{A}$ undergoes alkaline oxidative fusion to yield a dark green compound $\textbf{B}$ . Electrolytic oxidation of $\textbf{B}$ produces an intensely purple-colored compound $\textbf{C}$ . Separately, fusion of chromite ore ( $\ce{FeCr2O4}$ ) with $\ce{Na2CO3}$ in air yields a yellow compound $\textbf{D}$ .

Both $\textbf{C}$ and $\textbf{D}$ contain central transition metals in their highest oxidation states. Which of the following statements correctly identifies their spin-only magnetic moments ( $\mu_{eff}$ ), the origin of their intense colors, and the relative energy of their color-producing electronic transitions?

Options

A.
Both $\textbf{C}$ and $\textbf{D}$ have $\mu_{eff} = 0$ BM. Their colors arise from Ligand-to-Metal Charge Transfer (LMCT). The LMCT energy gap is smaller for $\textbf{C}$ than for $\textbf{D}$ because the higher oxidation state of the metal in $\textbf{C}$ lowers the energy of its empty d-orbitals.
B.
Both $\textbf{C}$ and $\textbf{D}$ have $\mu_{eff} = 0$ BM. Their colors arise from d-d transitions. The transition energy is higher for $\textbf{C}$ than for $\textbf{D}$ due to the greater tetrahedral crystal field splitting ( $\Delta_t$ ) induced by the +7 oxidation state.
C.
$\textbf{C}$ has $\mu_{eff} = 1.73$ BM and $\textbf{D}$ has $\mu_{eff} = 0$ BM. The color of $\textbf{C}$ is due to a Laporte-allowed d-d transition, while $\textbf{D}$ is colored via LMCT. The absorption energy for $\textbf{C}$ is lower than for $\textbf{D}$ .
D.
Both $\textbf{C}$ and $\textbf{D}$ have $\mu_{eff} = 0$ BM. Their colors arise from Metal-to-Ligand Charge Transfer (MLCT). The MLCT energy gap is larger for $\textbf{C}$ because the +7 charge strongly binds the d-electrons, preventing transfer to the oxygen ligands.

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Explanation

How the Examiner Thinks

Mechanism_Trap

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Concepts Behind This Question

Variable Oxidation State Stability|Color and Charge Transfer|Spin-Only Magnetic Moment|Magnetic Moment Quenching

Why This Question Is Tricky

Common Mistakes

The question tests the 'Chemical Exception' where highly colored transition metal complexes are assumed by students to derive their color from d-d transitions.

Here, the $d^0$ configuration dictates that d-d transitions are impossible, forcing the recognition of Ligand-to-Metal Charge Transfer (LMCT).

Furthermore, it tests a deep periodic trend: how oxidation state and effective nuclear charge affect the energy levels of empty d-orbitals.

Students often mistakenly assume that a higher oxidation state implies a larger energy gap (confusing LMCT gap with Crystal Field Splitting Energy, $\Delta_t$ , which does increase with oxidation state).

In LMCT, a higher oxidation state lowers the metal's d-orbital energy, decreasing the gap between the ligand p-orbitals and metal d-orbitals.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	D and F Block Elements - Class 12
Concept	Variable Oxidation State Stability
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free