$ \text{CuSO}_4$ is colorless but $\text{CuSO}_4\text{.5H}_2\text{O} $ is blue in color. Which of the following statements is incorrect about the color of $\text{CuSO}_4\text{.5H}_2\text{O} $?

presence of $ \text{ SO}_4^{2-}$ ion in coordination sphere imparts the color of $\text{CuSO}_4\text{.5H}_2\text{O} $

The correct answer is Option (3) → presence of $ \text{ SO}_4^{2-}$ ion in coordination sphere imparts the color of $\text{CuSO}_4\text{.5H}_2\text{O} $

Color in transition metal complexes arises due to d–d transitions caused by crystal field splitting in the presence of ligands.

What is d–d Transition?

In transition metal ions, the five d-orbitals split into two energy levels when ligands approach the metal ion (crystal field splitting). When light falls on the complex, an electron from a lower energy d-orbital absorbs energy and jumps to a higher energy d-orbital. This movement of an electron within the same d-subshell is called a d–d transition.

Why does it cause color?

• White light contains many wavelengths.
• Some wavelengths are absorbed for the d–d transition.
• The remaining transmitted/reflected light appears as color.

Example: $\text{Cu}^{2+}$ in $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$ absorbs red light $\rightarrow$ appears blue.

Conditions for d–d Transition

Colour due to d–d transition appears only if:

• d-orbitals are partially filled.
• Ligands cause crystal field splitting.

Hence:

$Sc^{3+} (d^0) \rightarrow$ colourless

$Zn^{2+} (d^{10}) \rightarrow$ colourless

$Cu^{2+} (d^9) \rightarrow$ coloured

Explanation

Option 1: In $CuSO_4 \cdot 5H_2O$ water acts as ligand

In hydrated copper sulphate, water molecules coordinate with $Cu^{2+}$ ion forming a complex. These water molecules act as ligands and influence the electronic environment.

Option 2: Crystal field splitting takes place in $CuSO_4 \cdot 5H_2O$

The interaction between $Cu^{2+}$ and water ligands causes splitting of d-orbitals into different energy levels.

Option 3: Presence of $SO_4^{2-}$ ion in coordination sphere imparts the color

Sulphate ion does not lie in the coordination sphere responsible for color. The color arises due to ligand field created by water molecules.

Option 4: d–d transition takes place in $CuSO_4 \cdot 5H_2O$

The blue color is due to electronic transition between split d-orbitals of $Cu^{2+}$ ion.