NEET 2026 ALL Q #1962
COMPETENCY BASED
APPLY
4 Marks 2026 NTA-RE-NEET-2026 MCQ SINGLE
The complex which has facial and meridional isomers is (Given: py = pyridine and en = $H_{2}N-CH_{2}-CH_{2}-NH_{2}$)
(A) $[Ni(en)_{2}(H_{2}O)_{2}]^{2+}$
(B) $[Cr(py)_{3}(Cl)_{3}]$
(C) $[Cr(H_{2}O)_{6}]^{3+}$
(D) $[Co(NH_{3})_{4}(H_{2}O)_{2}]^{3+}$
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Correct Answer: B

AI Tutor Explanation

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Detailed Solution

Step 1: Understanding Fac-Mer Isomerism

Facial (fac) and meridional (mer) isomerism is a type of geometric isomerism observed in octahedral complexes with the general formula MA3B3. In the fac isomer, three identical ligands occupy the corners of one triangular face of the octahedron. In the mer isomer, the three identical ligands lie on a meridian (a plane passing through the metal atom).

Step 2: Analyzing the Options

We need to identify which complex fits the MA3B3 pattern:

  • (A) $[Ni(en)_{2}(H_{2}O)_{2}]^{2+}$: This is an MA2B2 type (where en is bidentate). It shows cis-trans isomerism, not fac-mer.
  • (B) $[Cr(py)_{3}(Cl)_{3}]$: This is an MA3B3 type where M=Cr, A=py, and B=Cl. This complex can exist in both fac and mer forms.
  • (C) $[Cr(H_{2}O)_{6}]^{3+}$: This is an MA6 type. It is highly symmetrical and does not show geometric isomerism.
  • (D) $[Co(NH_{3})_{4}(H_{2}O)_{2}]^{3+}$: This is an MA4B2 type. It shows cis-trans isomerism, not fac-mer.

Step 3: Conclusion

Only the complex $[Cr(py)_{3}(Cl)_{3}]$ satisfies the structural requirement for facial and meridional isomerism.

Final Answer: B

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Pedagogical Audit
Bloom's Analysis: This is an APPLY question because the student must apply the structural criteria of coordination isomerism to identify the correct chemical formula.
Knowledge Dimension: CONCEPTUAL
Justification: The question tests the understanding of geometric isomerism principles in coordination chemistry rather than rote memorization of specific compounds.
Syllabus Audit: In the context of NEET, this is classified as COMPETENCY. It requires the student to categorize complex formulas based on their coordination geometry, a core skill for the Coordination Compounds unit.

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