Q.What kind of isomerism exists between $[Cr(H_2O)_6]Cl_3$ (violet) and $[Cr(H_2O)_5Cl]Cl_2 \cdot H_2O$ (greyish-green)?

The key idea is solvate isomerism (also called hydrate isomerism), where water molecules can be present inside the coordination sphere or as water of crystallisation.

Reasoning:

1. The violet compound has all six water molecules directly bonded to chromium: $[Cr(H_2O)_6]Cl_3$.
2. The greyish-green compound has five coordinated water molecules and one chloride inside the sphere, with the sixth water molecule outside as water of crystallisation: $[Cr(H_2O)_5Cl]Cl_2 \cdot H_2O$. …

Why this is not a trick — it’s about where the water lives

In coordination chemistry, isomers are compounds with the same overall formula but different arrangements of ligands around the metal. The key here is that both compounds have the same total composition: one chromium, six water molecules, three chloride ions. But the way these pieces are distributed between the inner coordination sphere (directly bonded to Cr) and the outer sphere (free ions or lattice water) is different.

The violet compound has all six water molecules directly attached to chromium. The greyish-green compound has only five water molecules attached; the sixth water is outside the coordination sphere, written as water of crystallisation. That is the entire difference.

Let’s check each option systematically.

1. What is the actual formula of each?

Write them with the coordination sphere inside square brackets and the counter-ions outside.

• Violet: $[Cr(H_2O)_6]Cl_3$

  → Coordination sphere: $[Cr(H_2O)_6]^{3+}$

  → Three $Cl^-$ ions outside.

• Greyish-green: $[Cr(H_2O)_5Cl]Cl_2 \cdot H_2O$

  → Coordination sphere: $[Cr(H_2O)_5Cl]^{2+}$

  → Two $Cl^-$ ions outside, plus one $H_2O$ molecule of crystallisation.

The overall molecular formula of both is the same: $CrCl_3 \cdot 6H_2O$.

2. Does the difference involve a change in the atom directly bonded to Cr?

Linkage isomerism (option i) occurs when a ligand can bind through two different atoms — e.g. $NO_2^-$ binding through N or O. Here, all ligands are either $H_2O$ or $Cl^-$, each of which binds only through one atom (O or Cl). No ambidentate ligand is present. So linkage isomerism is ruled out.

3. Does the difference involve swapping ions between inside and outside the sphere?

Ionisation isomerism (option iii) occurs when a counter-ion and a ligand exchange places, producing different ions in solution. For example, $[Co(NH_3)_5Br]SO_4$ vs $[Co(NH_3)_5SO_4]Br$. In our case, the exchange is between a water molecule and a chloride ion — but water is a neutral molecule, not an ion. The chloride ions do swap: one $Cl^-$ moves from outside to inside, but the other species moving is $H_2O$, not an ion. So this is not purely ionisation isomerism. (Some textbooks call it hydrate isomerism, a subset of solvate isomerism.)

4. Does the difference involve swapping entire coordination spheres? …

Method: Hydrate (Solvate) Isomerism Identification

This method is used when water molecules appear both inside and outside the coordination sphere in different isomers of the same overall formula.

Steps

1. Write the full molecular formula for both compounds (ignoring the dot for the hydrate):

   • $[Cr(H_2O)_6]Cl_3$ → $\text{Cr} \cdot 6\text{H}_2\text{O} \cdot 3\text{Cl}$
   • $[Cr(H_2O)_5Cl]Cl_2 \cdot H_2O$ → $\text{Cr} \cdot 6\text{H}_2\text{O} \cdot 3\text{Cl}$

   Both have the same overall formula: $\text{CrCl}_3 \cdot 6\text{H}_2\text{O}$.

2. Identify the coordination sphere in each:

   • First: $[Cr(H_2O)_6]^{3+}$ — all 6 water molecules are inside the sphere.
   • Second: $[Cr(H_2O)_5Cl]^{2+}$ — only 5 water molecules are inside; the sixth water is outside as water of crystallisation.

3. Check the type of isomerism:

   • The difference lies in how many water molecules are coordinated vs. present as solvent of crystallisation. …

Common Mistakes & How to Avoid Them

Mistake 1: Confusing solvate isomerism with hydrate isomerism

The error: Many students think "solvate isomerism" is a separate type from "hydrate isomerism." In fact, hydrate isomerism is a specific case of solvate isomerism where the solvent is water.

How to avoid: Remember the hierarchy:

• Solvate isomerism = general term (any solvent molecule)
• Hydrate isomerism = when the solvent is specifically water

Since both compounds contain water ($H_2O$), this is hydrate isomerism, which falls under solvate isomerism. Option (ii) solvate isomerism is correct.

Mistake 2: Misidentifying the isomerism type based on colour change

The error: Students see different colours (violet vs. greyish-green) and assume it must be linkage isomerism, because linkage isomers often have different colours.

How to avoid: Colour difference is not diagnostic — many types of isomerism produce colour changes. Focus on structural differences:

Here, water moves from inside the coordination sphere ($[Cr(H_2O)_6]^{3+}$) to outside as water of crystallisation ($Cl_2 \cdot H_2O$). That's solvate isomerism.

Mistake 3: Thinking it's ionisation isomerism

The error: Students see $Cl_3$ vs $Cl_2$ and think the chloride ions are being exchanged.

How to avoid: In ionisation isomerism, the anion in the coordination sphere swaps with a different anion outside. For example:

• $[Co(NH_3)_5Br]SO_4$ (gives $SO_4^{2-}$ in solution)
• $[Co(NH_3)_5SO_4]Br$ (gives $Br^-$ in solution)

Here, both compounds have chloride as the counterion — just different numbers. The key change is water, not chloride. So it's not ionisation isomerism.

Mistake 4: Confusing with coordination isomerism

The error: Students think the chromium coordination sphere changes.

How to avoid: Coordination isomerism occurs when both cation and anion are complex ions that exchange ligands. Example: …