Q.Give evidence that $[Co(NH_3)_5Cl]SO_4$ and $[Co(NH_3)_5(SO_4)]Cl$ are ionisation isomers.

Concept: Werner Coordination Theory — ionisation isomers exchange a ligand with the counter-ion, producing different ions in solution.

Reasoning:

1. In $[Co(NH_3)_5Cl]SO_4$, the chloride is coordinated to cobalt and sulphate is the free counter-ion. Dissolved in water, it gives $SO_4^{2-}$ ions, which precipitate with $Ba^{2+}$ as white $BaSO_4$.
2. In $[Co(NH_3)_5(SO_4)]Cl$, sulphate is coordinated and chloride is free. This isomer gives $Cl^-$ ions in solution, which precipitate with $Ag^+$ as white $AgCl$. …

The core idea: Werner’s coordination theory

Alfred Werner showed that in coordination compounds, a metal ion is surrounded by a fixed number of ligands in a primary (coordination) sphere. Ions outside this sphere are free to dissociate in solution. Ionisation isomers arise when the same set of atoms can be arranged so that a different ion is inside the coordination sphere versus outside. The two isomers have the same molecular formula but produce different ions when dissolved.

For the pair $\ce{[Co(NH3)5Cl]SO4}$ and $\ce{[Co(NH3)5(SO4)]Cl}$, the difference is simple: in the first, chloride is coordinated and sulfate is free; in the second, sulfate is coordinated and chloride is free. This swapping changes the electrical conductivity and, more importantly, the identity of the precipitate formed with appropriate reagents.

Step-by-step reasoning

1. Identify the coordination sphere in each isomer

   In $\ce{[Co(NH3)5Cl]SO4}$, the square brackets enclose the coordination sphere: $\ce{Co^{3+}}$ is bonded to five $\ce{NH3}$ molecules and one $\ce{Cl-}$ ligand. The sulfate ion $\ce{SO4^{2-}}$ lies outside, as a counterion.

   In $\ce{[Co(NH3)5(SO4)]Cl}$, the sphere contains $\ce{Co^{3+}}$ with five $\ce{NH3}$ and one $\ce{SO4^{2-}}$ ligand. Now chloride $\ce{Cl-}$ is the free counterion.

2. What happens when each isomer dissolves in water?

   The free ions dissociate completely.

   • Isomer A: $\ce{[Co(NH3)5Cl]SO4 -> [Co(NH3)5Cl]^{2+} + SO4^{2-}}$
   • Isomer B: $\ce{[Co(NH3)5(SO4)]Cl -> [Co(NH3)5(SO4)]^{+} + Cl-}$

   So the solution of isomer A contains free sulfate ions; the solution of isomer B contains free chloride ions.

3. Use a precipitation test to distinguish them

   Add a solution of barium chloride ($\ce{BaCl2}$) to each.

   • With isomer A: free $\ce{SO4^{2-}}$ reacts with $\ce{Ba^{2+}}$ to form a white precipitate of $\ce{BaSO4}$.
   • With isomer B: no free sulfate is present — the sulfate is bound inside the coordination sphere and does not react. No precipitate forms.

   Now add silver nitrate ($\ce{AgNO3}$) to fresh samples.

   • With isomer A: no free chloride — no precipitate of $\ce{AgCl}$.
   • With isomer B: free $\ce{Cl-}$ gives a white curdy precipitate of $\ce{AgCl}$.

4. Confirm the charges and conductivities …

Method: Conductivity & Precipitation Test for Ionisation Isomers

Concept: Werner’s coordination theory distinguishes between ionisable (outside coordination sphere) and non-ionisable (inside coordination sphere) groups. Ionisation isomers exchange a ligand inside the sphere with a counter-ion outside, giving different ions in solution.

Steps to prove $[Co(NH_3)_5Cl]SO_4$ and $[Co(NH_3)_5(SO_4)]Cl$ are ionisation isomers:

1. Write the dissociation equations

   • Isomer A: $[Co(NH_3)_5Cl]SO_4 \xrightarrow{H_2O} [Co(NH_3)_5Cl]^{2+} + SO_4^{2-}$
   • Isomer B: $[Co(NH_3)_5(SO_4)]Cl \xrightarrow{H_2O} [Co(NH_3)_5(SO_4)]^{+} + Cl^{-}$

2. Conductivity measurement

   • Isomer A gives 2 ions: $[Co(NH_3)_5Cl]^{2+}$ + $SO_4^{2-}$ — a 2:2-type electrolyte (a doubly charged ion pair).
   • Isomer B gives 2 ions: $[Co(NH_3)_5(SO_4)]^{+}$ + $Cl^{-}$ — a 1:1-type electrolyte (a singly charged ion pair).
   • Both give the same number of ions, but the ionic charges differ, so their molar conductivities differ (the 2+/2− pair conducts more strongly).

3. Precipitation test with $BaCl_2$ (test for free $SO_4^{2-}$)

   • Isomer A: White precipitate of $BaSO_4$ forms immediately because $SO_4^{2-}$ is free. …

Common Mistakes Students Make with Ionisation Isomers (Werner's Theory)

Mistake 1: Confusing the Counter-Ion with the Ligand

The error: Students often think the sulphate ($SO_4^{2-}$) is always a ligand or always a counter-ion. They fail to check where it appears in the formula.

Why it's wrong: In $[Co(NH_3)_5Cl]SO_4$, the sulphate is outside the square bracket — it is a free counter-ion. In $[Co(NH_3)_5(SO_4)]Cl$, the sulphate is inside the bracket — it is a ligand bonded to cobalt.

How to avoid: Always draw a box around the coordination sphere. Everything inside is a ligand; everything outside is a counter-ion. If the same ion appears in different positions, you likely have ionisation isomers.

Mistake 2: Thinking the Compounds Are Identical

The error: Students see the same atoms ($Co$, $NH_3$, $Cl$, $SO_4$) and assume the compounds are the same.

Why it's wrong: The arrangement of ions between the coordination sphere and the free counter-ion changes. This changes which ions are released in solution.

How to avoid: Write the dissociation equations:

• $[Co(NH_3)_5Cl]SO_4 \rightarrow [Co(NH_3)_5Cl]^{2+} + SO_4^{2-}$

  (gives sulphate ions in solution)

• $[Co(NH_3)_5(SO_4)]Cl \rightarrow [Co(NH_3)_5(SO_4)]^{+} + Cl^{-}$

  (gives chloride ions in solution)

Key result: Different ions are released — this is the evidence for ionisation isomerism.

Mistake 3: Forgetting to Test with a Precipitating Agent

The error: Students state the isomers are different but don't mention how to prove it experimentally.

Why it's wrong: Werner's theory requires experimental evidence. Simply writing formulas is not enough.

How to avoid: Remember the BaCl₂ test and AgNO₃ test:

Key result: The first isomer gives $BaSO_4$ precipitate; the second gives $AgCl$ precipitate. This is direct evidence of different free ions.

Mistake 4: Confusing Ionisation Isomerism with Linkage Isomerism

The error: Students think this is about how $SO_4$ binds (through O or S) — that's linkage isomerism.

Why it's wrong: In ionisation isomerism, the position of the ion (inside vs outside the coordination sphere) changes. The bonding mode of the ligand is irrelevant here.

How to avoid:

• Ionisation isomerism: Same atoms, different free ions. …