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

Solution

Chemistry ma’am · 3 steps

First find the metal's d-electron count: with zero d electrons, there is no d-d transition to see.

1. The rule. A d-d transition moves an electron from a lower d orbital to a higher one. That needs a partly filled d subshell, so the metal ion must have between 1 and 9 d electrons.

   Compound	Metal oxidation state	d electrons	Source of colour
   K₂Cr₂O₇	Cr +6	3d⁰	charge transfer (orange)
   CuSO₄.5H₂O	Cu +2	3d⁹	d-d transition (blue)
   KMnO₄	Mn +7	3d⁰	charge transfer (purple)
   K₂CrO₄	Cr +6	3d⁰	charge transfer (yellow)

2. Charge transfer. In Cr₂O₇²⁻, MnO₄⁻ and CrO₄²⁻ the metal has no d electrons. Their colour comes from an electron jumping from oxygen to the metal, called ligand-to-metal charge transfer.
3. So. Only (b) fits. In [Cu(H₂O)₄]²⁺ the 3d⁹ ion absorbs red-orange light through a d-d transition and looks blue.

The idea to keep. Count the d electrons first. If the metal is d⁰ or d¹⁰, no d-d transition is possible, so any colour it shows must come from charge transfer.

Answer from the NTA final answer key. Solution by PrepWiser.