A solution `[Ni(H_2O)_6]^(2+)` is green while a solution of `[Ni(CN)_4]^(2-)` is colourless. Explain.

Explanation

The configuration of `Ni^(2+)` ions is `[Ar]3d^(8),4s^(0)`

In `[Ni(H_(2)O)_(6)]^(2+),H_(2)O` molecules are weak field ligands, they do not cause electrons. Thus, d-d transition takes place due to absorption of radiation corresponding to red light and the emission of complementary green colour occurs. Hence, it is green.
In `[Ni(CN)_(4)]^(2-),CN^(-)` are strong ligands so in the presence of `CN^(-)` ions, the two unpaired electrons in the 3d-orbital pair up. Hence, there is no unpaired electron and no transition. Hence, it is colourless.