The complex that can show fac- and mer-isomers is:

🧠 fac/mer Requires $\mathrm{[MA_3B_3]}$

For an octahedral complex to display fac/mer isomers, it must have a 3+3 split of two distinct monodentate ligand types — three of one, three of another.

🗺️ Scan Each Option

| Complex | Pattern | fac/mer? | |---|---|---| | (1) $[\mathrm{Co(NH_3)_4Cl_2}]^+$ | $\mathrm{[MA_4B_2]}$ — 4+2 | only cis/trans | | (2) $[\mathrm{Pt(NH_3)_2Cl_2}]$ | square-planar 2+2 | only cis/trans | | (3) $[\mathrm{CoCl_2(en)_2}]$ | 2 bidentate + 2 monodentate | cis/trans + chirality | | (4) $[\mathrm{Co(NH_3)_3(NO_2)_3}]$ | $\mathrm{[MA_3B_3]}$ — 3+3 | fac/mer ✓ |

⚡ The "3+3 = fac/mer" Pattern

Whenever you see exactly 3 of one ligand and 3 of another in an octahedral complex, you've found a fac/mer pair. No other arrangement supports it.

⚠️ Bidentate Ligands Don't Form fac/mer

$[\mathrm{Co(en)_3}]$ has 6 donor atoms but only Δ/Λ optical isomers — not fac/mer. fac/mer requires three separate, identical monodentate ligands of one type.

$\boxed{\text{Answer: (4) } [\mathrm{Co(NH_3)_3(NO_2)_3}]}$