JEE Main 2020 · 8 January · morning · Single correct · +4 / −1
The complex that can show fac- and mer-isomers is :
- (a) [CoCl₂(en)₂]
- (b) [Pt(NH₃)₂Cl₂]
- (c) [Co(NH₃)₃(NO₂)₃]
- (d) [Co(NH₃)₄Cl₂]⁺
Answer and solution
Answer (c) [Co(NH₃)₃(NO₂)₃]
Solution
Chemistry ma’am · 3 steps
Fac and mer need an octahedron holding three of one ligand: look for the MA₃B₃ pattern.
- Fac versus mer. In an octahedral MA₃B₃ complex, the three A ligands can sit on one triangular face (fac). They can also sit along a line through the metal, a meridian (mer). This needs six coordination and exactly three of each ligand.
Complex Geometry Type Isomerism shown [CoCl₂(en)₂] octahedral M(AA)₂B₂ cis-trans (cis is also optical) [Pt(NH₃)₂Cl₂] square planar MA₂B₂ cis-trans only [Co(NH₃)₃(NO₂)₃] octahedral MA₃B₃ fac and mer [Co(NH₃)₄Cl₂]⁺ octahedral MA₄B₂ cis-trans only - Why only one fits. Two identical ligands can only be cis or trans to each other. You need a set of three identical ligands before the face-versus-meridian choice exists.
- So. Only (c), [Co(NH₃)₃(NO₂)₃], is an octahedral MA₃B₃ complex, so it is the one that shows fac and mer isomers.
The idea to keep. Fac and mer are about three identical ligands in an octahedron. A pair of identical ligands gives only cis and trans, so first look for the MA₃B₃ pattern.
Answer from the NTA final answer key. Solution by PrepWiser.