question_answer 30)

A solution of [Ni(H2O)6]2+ is green but a solution of [Ni(CN)4]2– is colourless. Explain.

Answer:

In [Ni(H2O)6]2+ , Ni is in +2 state with the configuration 3d8, i.e., it has two unpaired electrons which do not pair up in the presence of the weak H2O ligand. Hence, it is coloured. For d–d transition, red light is absorbed and complementary light emitted is green. In case of [Ni(CN)4]2– , Ni is again in +2 stat with the configuration 3d8 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.