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Question #44894

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The Question

Actinoids in the $+4$ oxidation state exhibit coordination chemistry that significantly deviates from $3d$ transition metals. When an aqueous solution of $Th^{4+}$ is treated with one equivalent of $EDTA^{4-}$ at $298\text{ K}$ , a stable, neutral mononuclear complex $[Th(EDTA)(H_2O)_n]$ is isolated. Which of the following correctly identifies the number of coordinated water molecules ( $n$ ), the total coordination number (CN) of Thorium, and the primary governing principle for this structure?

Options

A.
$n = 0$ , CN = 6; The hexadentate EDTA completely encapsulates the metal, forming a sterically saturated octahedral complex governed by Crystal Field Theory.
B.
$n = 2$ , CN = 8; The complex adopts a square antiprismatic geometry to maximize covalent orbital overlap with the directional $5f$ orbitals.
C.
$n = 3$ , CN = 9; The large ionic radius of $Th^{4+}$ minimizes steric crowding, allowing expansion of the coordination sphere driven predominantly by electrostatic interactions.
D.
$n = 3$ , CN = 9; The complex forms specifically to obey the 18-electron rule, as the 9 coordinate bonds provide exactly 18 valence electrons to the $Th^{4+}$ ion.

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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

Actinoid Oxidation State Variety|Ligand Denticity Variations|Coordination Isomerism

Why This Question Is Tricky

Common Mistakes

The question tests the 'Chemical Exception' of high coordination numbers in f-block elements.

Students are conditioned by 3d transition metal chemistry to assume EDTA forms a saturated, octahedral, 6-coordinate complex (Option A).

The trap further exploits a coincidental alignment with the 18-electron rule (Option D), which advanced students often misapply to f-block coordination complexes.

The true hidden variable is the transition from CFT/covalent dominance in the d-block to electrostatic/steric dominance in the f-block, allowing large actinoids like Th(IV) to easily accommodate CN=8, 9, or even 12.

Quick Facts About This Question

Question Information

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