1. In [Ni(H2O)6]2+ , Ni is in +2 oxidation state with the configuration 3d8 , i.e., it has two unpaired electrons which do not pair up in the presence of weak H2O ligand. Hence, it is coloured. The d – d transtion absorbs red light and the complementary light emitted is green.
2. In the case of [Ni(CN4)]2- Ni is again in +2 oxidation state with the configuration 3d8, but in the presence of strong CN- ligand the two impaired electrons in the 3d orbitals pair up. Thus there is no unpaired electron present. Hence it is colourless. Therefore, a solution of [Ni(H2O)6]2+ is green, whereas a solution of [Ni(CN4)]2-is colourless.
