The complex cation which has two isomers is:

🧠 Look for Linkage Isomerism

A complex has 2 isomers when:

• It has an ambidentate ligand (e.g. $\mathrm{NO_2^-}$ — N-bonded "nitro" vs O-bonded "nitrito"), OR
• It has $MA_4B_2$ / $MA_3B_3$ geometric flexibility.

For pentammine cobalt complexes with one differing ligand, only ambidentate ligands give isomers.

🗺️ Evaluate Each

| Complex | Isomers? | Why | |---|---|---| | $[\mathrm{Co(H_2O)_6}]^{3+}$ | No | Homoleptic — only 1 isomer | | $[\mathrm{Co(NH_3)_5 Cl}]^{2+}$ | No | $MA_5B$ — no geometric/linkage isomers | | $[\mathrm{Co(NH_3)_5 NO_2}]^{2+}$ | 2 | $\mathrm{NO_2^-}$ ambidentate: nitro (N-bonded) vs nitrito (O-bonded) | | $[\mathrm{Co(NH_3)_5 Cl}]^+$ | No | Same as above (just different OS) |

⚡ Common Ambidentate Ligands

| Ligand | Two donor atoms | |---|---| | $\mathrm{NO_2^-}$ | N (nitro) or O (nitrito) | | $\mathrm{SCN^-}$ | S (thiocyanato) or N (isothiocyanato) | | $\mathrm{CN^-}$ | C (cyano) or N (isocyano) |

⚠️ MA₅B Type Has No Geometric Isomers

In an octahedral $[\mathrm{MA_5B}]$, all five A's are equivalent (no cis/trans distinction). Only ambidentate B can produce 2 isomers.

$\boxed{\text{Answer: (3) } [\mathrm{Co(NH_3)_5 NO_2}]^{2+}}$