Question

How would you account for the following?

[Ti(H₂O)₆]³⁺ is coloured, while [Sc(H₂O)₆]³⁺ is colourless.

Answer 1

The difference in colour between the complexes [Ti(H₂O)₆]³⁺ and [Sc(H₂O)₆]³⁺ is due to the presence or absence of d–d electronic transitions in the metal ions. In [Ti(H₂O)₆]³⁺, the titanium ion is in the +3 oxidation state, with an electronic configuration of 3d¹. When this complex is formed, the d-orbitals split into two energy levels (t₂g and e_n) due to the octahedral field created by the surrounding water ligands. The single d-electron can absorb visible light and undergo a d–d transition between these energy levels, resulting in the complex appearing coloured. In contrast, [Sc(H₂O)₆]³⁺ contains scandium in the +3 oxidation state, which has a 3d⁰ configuration meaning it has no d-electrons available for such transitions. As a result, there is no absorption of visible light, and the complex remains colourless. Therefore, the availability of d-electrons for electronic transitions directly influences the colour of transition metal complexes.