Question

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

Answer 1

In\( [Ni(H_2O)_6]^{ 2+}\) ,\(H_2\ddot{O}\) is a weak field ligand. Therefore, there are unpaired electrons in \(Ni^{2+}\). In this complex, the delectrons from the lower energy level can be excited to the higher energy level i.e., the possibility of \(d - d\) transition is present. Hence, \(Ni(H_2O)_6]^{ 2+}\) is coloured. In \([Ni(CN)_4] ^{2-} \), the electrons are all paired as \(CN^{ -}\) is a strong field ligand. Therefore, d-d transition is not possible in \([Ni(CN)_4]^{ 2-}\) . Hence, it is colourless.