Question

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

Answer 1

In [Ni(H₂O)₆]²⁺, is a weak field ligand. Therefore, there are unpaired electrons in Ni²⁺. In this complex, the d electrons 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(H₂O)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.