Explain why cuprous chloride `(CuCI)` is colourless white cupric shloride `(CuCl_2)` in blue? Or `TiCl_3` is coloured while `TiCl_4`1 is colourless?

Explanation

In `CuCl`, `Cu^(o+)` has fully-filled d-subshell. Hence, it cannot absorb energy for d-d transition. Therefore, it is colourless. In `CuCl_2`, `Cu^(2+)` has `3d^9` configuration i.e., one unpaired electron is present which can absorb energy for d-d transition. Hence, it is coloured.
Similarly, `Ti^(3+)` has `3d^1` and `Ti^(4+)` has emptly 3d subshell. Hence, `TiCl_3` is coloured while `T9Cl_4` is colourless.