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Question
How would you account for the following?
[Ti(H2O)6]3+ is coloured, while [Sc(H2O)6]3+ is colourless.
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Solution
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 en) 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.
