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)_6]^(2-)` Ni is in +2 state with the configuration `2d^8`, i.e., it has two unpaired electrons which do not pair up in the presence of the weak `H_2 O` ligand. So, 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^8` but in presence of the strong `CN^-` ligand, the two unpaired electrons in the 3d-orbitals pair up. Hence, there is no unpaired electron present, Hence, it is colourless.