Give evidence that $[Co(NH_3)_5Cl]SO_4$ and $[Co(NH_3)_5SO_4]Cl$ are ionization isomers.

**OR**

$[NiCl_4]^{2-}$ is paramagnetic while $[Ni(CO)_4]$ is diamagnetic though both are tetrahedral. Explain why.

Question

Give evidence that $[Co(NH_3)_5Cl]SO_4$ and $[Co(NH_3)_5SO_4]Cl$ are ionization isomers.

**OR**

$[NiCl_4]^{2-}$ is paramagnetic while $[Ni(CO)_4]$ is diamagnetic though both are tetrahedral. Explain why.

Accepted Answer

>

> [!TLDR]
> Ionization isomers have the same overall formula but differ in which ion sits inside the coordination sphere and which is the free counter-ion; this is proved experimentally because each isomer gives a precipitate with a different reagent.
>
> **Ionization isomerism and the experimental evidence**
>
> Ionization isomers are complexes that have the identical molecular formula but produce **different ions in solution**, because a ligand and the counter-ion have effectively swapped places between the coordination sphere and the ionizable (outer) position.
>
> - $[Co(NH_3)_5Cl]SO_4$: here $Cl^-$ is coordinated (inside the bracket) and $SO_4^{2-}$ is the free counter-ion. In solution:
> $$[Co(NH_3)_5Cl]SO_4 \rightarrow [Co(NH_3)_5Cl]^{2+} + SO_4^{2-}$$
> Adding $BaCl_2(aq)$ gives an **immediate white precipitate of $BaSO_4$** (free $SO_4^{2-}$ is available), but adding $AgNO_3(aq)$ gives **no immediate precipitate** of $AgCl$ (the $Cl^-$ is locked inside the coordination sphere).
>
> - $[Co(NH_3)_5SO_4]Cl$: here $SO_4^{2-}$ is coordinated and $Cl^-$ is the free counter-ion:
> $$[Co(NH_3)_5SO_4]Cl \rightarrow [Co(NH_3)_5SO_4]^{+} + Cl^-$$
> Adding $AgNO_3(aq)$ now gives an **immediate white precipitate of $AgCl$**, while $BaCl_2(aq)$ gives **no immediate precipitate** of $BaSO_4$.
>
> This contrasting precipitation behaviour — each compound reacting instantly with only one of the two test reagents — is the direct chemical evidence that the two compounds, despite having the same overall formula, are genuinely different compounds (ionization isomers), because different ions are released into solution.
> …

Q.Give evidence that $[Co(NH_3)_5Cl]SO_4$ and $[Co(NH_3)_5SO_4]Cl$ are ionization isomers. OR $[NiCl_4]^{2-}$ is paramagnetic while $[Ni(CO)_4]$ is diamagnetic though both are tetrahedral. Explain why.

Concept understanding — Structural Isomerism in Coordination Compounds

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