A solution of `[Ni(H_(2)O)_(6)]^(2+)` is green but a solution of `[Ni(CN)_(4)]^(2-)` is colourless. Explain.

Explanation

`[Ni(H_(2)O)_(6)]^(2+), Ni^(2+)` has unpaired electrons as `H_(2)O` in a weak-field ligand. So, the possibility of d-d transition is present here, i.e.,, `[Ni(H_(2)O)_(6)]^(2+)` is coloured (complementary colour : green ). But in `[Ni(CN)_(4)]^(2-)`, the electrons are all paired up as `CN^(-)` is a strong-field ligand. Therefore d-d transition is not possible and hence, it is colourless