Question

Which of the following compounds show colour due to d-d transition?

• A. \( \text{CuSO}_4.5\text{H}_2\text{O} \)
• B. \( \text{K}_2\text{Cr}_2\text{O}_7 \)
• C. \( \text{K}_2\text{CrO}_4 \)
• D. \( \text{KMnO}_4 \)

Answer 1

The Correct Option is A

To determine which compound shows color due to d-d transition, we must understand the mechanism behind such color formation. The d-d transition occurs in complexes of transition metals, where electrons jump from one d-orbital to another. This transition requires partially filled d-orbitals and occurs in compounds with transition metals.

Now, let's analyze each given compound: 

1. \(\text{CuSO}_4.5\text{H}_2\text{O}\): This compound, known as copper(II) sulfate pentahydrate, contains copper in the +2 oxidation state. The electronic configuration of Cu²⁺ is 3d⁹, which has unpaired electrons in the d-orbitals, making d-d transitions possible. As a result, this compound exhibits a characteristic blue color due to d-d transition.
2. \(\text{K}_2\text{Cr}_2\text{O}_7\) and \text{K}_2\text{CrO}_4: These compounds, potassium dichromate and potassium chromate, respectively, have chromium in the +6 oxidation state, with no d-electrons left for d-d transitions. The color in these compounds is due to charge transfer rather than d-d transitions.
3. \(\text{KMnO}_4\): Potassium permanganate contains manganese in the +7 oxidation state, where no d-electrons can engage in d-d transitions. The color is explained by charge transfer transitions.

Thus, the only compound in the given options showing color due to d-d transition is \(\text{CuSO}_4.5\text{H}_2\text{O}\).

Answer 2

Explanation: CuSO$_4$ $\cdot$ 5H$_2$O (copper(II) sulfate pentahydrate) contains the Cu$^{2+}$ ion, which has a partially filled $d$-orbital. The electronic configuration of Cu$^{2+}$ is [Ar] $3d^9$. In an aqueous environment, the Cu$^{2+}$ ion forms a complex with water molecules, creating a distorted octahedral geometry. The $d$-orbitals of the Cu$^{2+}$ ion split into two energy levels due to the ligand field created by the surrounding water molecules. The absorption of visible light causes electrons to transition between these $d$-orbital energy levels ($d$-$d$ transitions), resulting in the characteristic blue colour of the compound.
Other Options: K$_2$Cr$_2$O$_7$ (potassium dichromate), K$_2$CrO$_4$ (potassium chromate), and KMnO$_4$ (potassium permanganate) show colour due to charge transfer transitions, not $d$-$d$ transitions. In these compounds, the chromate and permanganate ions are highly coloured due to the transfer of electrons between the metal and oxygen atoms.
Conclusion: The compound CuSO$_4$ $\cdot$ 5H$_2$O shows colour due to $d$-$d$ transitions.