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Question #44601

Difficulty: 8/10 | 🟢 Free

The Question

Consider the tetrahedral coordination complexes $\ce{[CoCl4]^{2-}}$ (Complex X) and $\ce{[Fe(CO)4]^{2-}}$ (Complex Y) at 298 K. Determine the Effective Atomic Number (EAN), the number of unpaired electrons ( $n$ ), and the Crystal Field Stabilization Energy (CFSE) for Complex X and Complex Y, respectively.

(Format: X: (EAN, $n$ , CFSE) ; Y: (EAN, $n$ , CFSE))

Options

A.
X: (33, 3, $-1.2 \Delta_t$ ) ; Y: (36, 0, 0)
B.
X: (33, 3, $-0.8 \Delta_t$ ) ; Y: (36, 0, $-2.4 \Delta_t$ )
C.
X: (35, 1, $-1.2 \Delta_t$ ) ; Y: (34, 2, $-0.6 \Delta_t$ )
D.
X: (36, 0, 0) ; Y: (36, 0, $-2.4 \Delta_t$ )

Check Your Answer

Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Mechanism_Trap

Don't get tricked by the process! Learn the mechanical traps used in competitive exam questions.

Concepts Behind This Question

EAN Rule (Sidgwick's Rule)|Tetrahedral Splitting Patterns|Crystal Field Splitting (Octahedral)

Why This Question Is Tricky

Common Mistakes

The question tests the intersection of the EAN rule, Crystal Field Theory (CFT), and spin-pairing energy in tetrahedral complexes.

A primary trap is the misapplication of octahedral CFSE formulas to tetrahedral geometries.

For instance, students might calculate a CFSE of $-0.8 \Delta_t$ for a $d^7$ system by using the octahedral high-spin formula ( $-0.4 \times 5 + 0.6 \times 2 = -0.8$ ).

Another critical trap is the 'Strong Field Ligand Illusion': students assume that because CO is a strong-field ligand, $\ce{[Fe(CO)4]^{2-}}$ must have a large negative CFSE (like $-2.4 \Delta_t$ ), completely overlooking that Fe(-II) is a $d^{10}$ system where CFSE is mathematically zero regardless of ligand strength.

Finally, it highlights that stable complexes like $\ce{[CoCl4]^{2-}}$ can violate the EAN rule while being thermodynamically favored due to their high-spin CFSE stabilization.

Quick Facts About This Question

Question Information

Subject	Chemistry
Chapter	Coordination Compounds - Class 12
Concept	EAN Rule (Sidgwick's Rule)
Difficulty	8
Cognitive Load	7
Target Solve Time	160 sec
Access	Free