JEE Main 2025 · 23 January · morning · Single correct · +4 / −1
The complex that shows Facial - Meridional isomerism is :
- (a) [Co(en)₂ Cl₂]⁺
- (b) [Co(NH₃)₃ Cl₃]
- (c) [Co(NH₃)₄ Cl₂]⁺
- (d) [Co(en)₃]³⁺
Answer and solution
Answer (b) [Co(NH₃)₃ Cl₃]
Solution
Chemistry ma’am · 3 steps
Fac-mer needs an MA₃B₃ octahedron: three of one ligand, three of another.
- Fac and mer. In an octahedral MA₃B₃ complex, the three A ligands can sit on one triangular face (fac). They can also lie in one plane through the metal, with two of them opposite each other (mer). Neither arrangement can be rotated into the other, so they are isomers.
Complex Type Isomerism shown [Co(en)₂Cl₂]⁺ M(AA)₂B₂ cis-trans (and optical for cis) [Co(NH₃)₃Cl₃] MA₃B₃ fac-mer [Co(NH₃)₄Cl₂]⁺ MA₄B₂ cis-trans only [Co(en)₃]³⁺ M(AA)₃ optical only - Check the others. In (a) and (c) there are only two Cl⁻, so the only question is whether they are next to each other or opposite each other. That gives cis-trans, not fac-mer. In (d) the three identical en ligands give mirror-image forms, not fac-mer.
- So. Only [Co(NH₃)₃Cl₃] has three NH₃ and three Cl⁻ around the octahedron. Option (b) shows facial-meridional isomerism.
The idea to keep. Spot fac-mer by the formula: an octahedral MA₃B₃ pattern. Two of a kind means cis-trans. Three identical chelates mean optical isomers.
Answer from the NTA final answer key. Solution by PrepWiser.