Which of the following complex has a possibility to exist as meridional isomer?

🧠 fac vs mer Needs MA₃B₃ Octahedral

A meridional (mer) isomer requires an octahedral $\mathrm{[MA_3B_3]}$ pattern: three identical ligands of one type and three of another.

• fac ("facial"): the three identical ligands occupy a triangular face of the octahedron (all 90° apart).
• mer ("meridional"): the three identical ligands lie on a meridian (two trans, one cis to both).

So we need a complex with 3+3 monodentate ligand split.

🗺️ Scan the Options

• (1) $[\mathrm{Co(NH_3)_3(NO_2)_3}]$ → 3 + 3 monodentates → fac/mer pair exists ✓
• (2) $[\mathrm{Pt(NH_3)_2Cl_2}]$ → 2 + 2 (square-planar) → only cis/trans, no fac/mer ✗
• (3) $[\mathrm{Co(en)_2Cl_2}]$ → 2 bidentate + 2 monodentate → cis/trans (and chirality), but no fac/mer ✗
• (4) $[\mathrm{Co(en)_3}]$ → 3 bidentate → only Δ/Λ, no fac/mer ✗

⚡ The fac/mer Trigger

When you see octahedral $\mathrm{[MA_3B_3]}$ — that's the only arrangement that gives fac/mer. Memorise:

| Pattern | Isomers possible | |---|---| | $\mathrm{[MA_4B_2]}$ | cis / trans | | $\mathrm{[MA_3B_3]}$ | fac / mer | | $\mathrm{[M(LL)_3]}$ | Δ / Λ | | $\mathrm{[M(LL)_2X_2]}$ | cis (chiral) / trans |

⚠️ Don't Confuse fac/mer With cis/trans

cis/trans applies to $\mathrm{[MA_4B_2]}$ (2 identical out of 6). fac/mer applies to $\mathrm{[MA_3B_3]}$ (3 identical out of 6). Different patterns.

$\boxed{\text{Answer: (1) } [\mathrm{Co(NH_3)_3(NO_2)_3}]}$