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

Explanation

`H_(2)O` is a weak ligand. 3d electrons of `Ni^(2+)` do not pair up. The unpaired electrons absorb red wavelength of white light for `d-d` transition and transmitted colour (complementary colour) is green. Hence, the solution of `[Ni(H_(2)O)_(6)]^(2+)` is green.
`CN^(-)` is a strong ligand. The 3d electrons of `Ni^(2+)` pair up. There is no unpaired electron left to absorb energy. Hence, solution of `[Ni(CN)_(4)]^(2+)` is colourless.