Question

What is the colour of transition metal ions having completely filled and empty d-orbitals?

Answer 1

1. Transition metal ions with completely filled d-orbitals (e.g., Cu⁺, Zn²⁺) are colourless because no vacant d-orbitals are available for electron promotion (no d-d transitions occur).
2. Those with completely empty d-orbitals (e.g., Sc³⁺, Ti⁴⁺) are also colourless because there are no electrons to undergo d-d transitions.
3. The colour in transition metal ions arises from d-d electronic transitions, which are absent in these two cases, making such ions colourless.

Hence, ions with fully filled or completely empty d-orbitals show no colour in their compounds.