Question

Why is a solution of Ni(H₂O)₆²⁺ green while a solution of Ni(CN)₄²⁻ is colourless? (At. No. of Ni = 28)

Hint:

The color of a complex is influenced by the strength of the ligand and the extent of splitting of the metal’s d-orbitals.

Answer 1

The color of a coordination compound is largely influenced by the ligand field and the d-electron transitions of the metal ion. In the case of Ni(H₂O)₆²⁺, water is a weak field ligand that causes a small splitting of the d-orbitals in Ni²⁺. This allows the absorption of light in the visible spectrum, giving the solution a green color.

In contrast, in Ni(CN)₄²⁻, cyanide is a strong field ligand that causes a large splitting of the d-orbitals in Ni²⁺, leading to no available electronic transitions in the visible region. As a result, the solution of Ni(CN)₄²⁻ is colorless because no visible light is absorbed.