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

Explanation

In `[Ni(H_(2)O_(2))_(6)]^(2+)`. Ni is in +2 state with the configuration `3d^(8)` i.e., it has two unpaired electrons which do not pair up in the presence of the weak `H_(2)O` ligand. Hence, it is coloured. The d-d transition, absorbs red light and the complementary light emitted is green
In case of `[Ni(CN)_(4)]^(2-)` Ni is again in +2 state with the configuration `3d^(@)` but in presence of the strong CN ligand, the two unpaired electrons in the 3d orbitals pair up. Thus, there is no unpaired electron present. Hence, it is colourless