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Question #44299

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The Question

A $3d$ -block transition metal $M$ exhibits its largest relative jump in successive ionization energies between $IE_7$ and $IE_8$ . In its highest oxidation state, $M$ forms an intensely colored, diamagnetic tetrahedral oxoanion $[MO_4]^z$ . Let $\Delta E$ be the energy of the primary electronic transition responsible for this color. If $M$ is replaced by the element $M'$ immediately preceding it in the $3d$ series (which forms an isostructural oxoanion $[M'O_4]^{z'}$ in its highest oxidation state), how does the transition energy $\Delta E$ change, and what is the fundamental origin of the color in $[MO_4]^z$ ?

Options

A.
$\Delta E$ increases; the color of $[MO_4]^z$ arises from Ligand-to-Metal Charge Transfer (LMCT) into the empty $3d$ orbitals.
B.
$\Delta E$ decreases; the color of $[MO_4]^z$ arises from Ligand-to-Metal Charge Transfer (LMCT) into the empty $3d$ orbitals.
C.
$\Delta E$ increases; the color of $[MO_4]^z$ arises from a spin-allowed $d-d$ transition due to a drastically increased crystal field splitting ( $\Delta_t$ ).
D.
$\Delta E$ decreases; the color of $[MO_4]^z$ arises from Metal-to-Ligand Charge Transfer (MLCT) driven by the high effective nuclear charge.

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Correct Answer

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Explanation

How the Examiner Thinks

Mechanism_Trap

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Concepts Behind This Question

Successive Ionization Energies|Crystal Field Splitting (Octahedral)|Color and Charge Transfer

Why This Question Is Tricky

Common Mistakes

This question tests two major exceptions/hidden variables in coordination chemistry.

First, it challenges the heuristic that transition metal colors are exclusively due to $d-d$ transitions.

Students must recognize that in high oxidation states (achieved after successive ionizations), the $d^0$ configuration precludes $d-d$ transitions, making LMCT the sole source of color.

Second, it tests the counter-intuitive trend of LMCT energy gaps: while a higher oxidation state drastically increases Crystal Field Splitting ( $\Delta_o$ or $\Delta_t$ ), it actually decreases the LMCT energy gap ( $\Delta E$ ) because the increased effective nuclear charge pulls the empty metal $d$ -orbitals down in energy, closer to the ligand $p$ -orbitals.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Classification of Elements and Periodicity in Properties - Class 11 Part 1
Concept	Successive Ionization Energies
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free