`[Ti(H_(2)O)_(6)]^(3+)` is coloured while `[Sc(H_(2)O)_(6)]^(3+)` is colourless. Why?

Explanation

In `[Ti(H_(2)O)_(6)]^(3+), Ti^(3+)` ion has one electron in the d-subshell (lower energy `t_(2g)` d-orbitals, has the configuration `t_(2g)^(1)e_(g)^(0)`) which can absorb light in the visible region resulting into d-d transition (jump into higher energy `e_(g)` orbitals). As a result, green and yellow portions of light are absorbed. The complex has the complementary colour, viz, purple. In `[Sc(H_(2)O)_(6)]^(3+)`, `Sc^(3+)` has no d-electron. Hence, no light is absorbed for d-d transition. Hence, it is colourless.