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

Explanation

In `[Ni(H_2O)_6]^(2+)`, Ni is in +2 slate with the configuration `3 d^8` having two unpaired electrons which do not pair up in the presence of the weak `H_2O` 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^8` 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.