Question

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

Answer 1

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In [Ni(H2O)6]2+,  H2Ö  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 - dtransition 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.

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