Explain why `[Ti (H_2O)_6]^(3+)` is violet while `[Ti (H_2O)_6]^(4+)` is colourless.

Explanation

`Ti` has electrónic configuration : `[Ar]^(18) 3d^2 4s^2`

With one electron in 3d-Subshell in `Ti^(3+)`, d-d transitions are possible and hence it is coloured with electron 3d-subshell

In `Ti^(4+)`, d-d transitions are not possible and hence it is colourless.