Why a solution of `[Ni(H_(2)O)_(6)]^(2+)` is green while a solution of `[Ni(CN)_(4)]^(2-)` is colourless? (At. No. of Ni = 28)

Explanation

In `[Ni(H_(2)O)_(6)]^(2+), Ni` is in `+2` oxidation state with the configuration `3d^(8) 4s^(0),` i.e., it has two unpaired electrons which do not pair up in the presence of weak `H_(2)O` ligand. The `d-d` transition absorbs red light and the complementary green light is emitted.

On the other hand, in `[Ni(CN)_(4)]^(2-)`, Ni is again in `+2` oxidation state with the electronic configuration `3d^(8)`. In the presence of strong `CN^(-)` ligand, the two unpaired electron in `[Ni(CN)_(4)]^(2-)` therefore the complex is colourless.