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+)` ion is
`Ni^(2+)=[Ar]3d^(8), 4s^(0)`

In `[Ni(H_(2)O)_(6)]^(2+), H_(2)O` molecules are weak field ligands, they do not cause electron pairing. As a result, the complex has two unpaired 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 nó unpaired electron and no transition. Hence, it is colourless.