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

To solve the problem, we need to explain why a solution of Ni(H₂O)₆²⁺ is green while a solution of Ni(CN)₄²⁻ is colorless.

1. Understanding the Complexes:
Nickel(II) ions (Ni²⁺) can form different coordination complexes depending on the ligands involved. In the case of Ni(H₂O)₆²⁺, water molecules act as ligands, while in Ni(CN)₄²⁻, cyanide ions (CN⁻) act as ligands.

2. Crystal Field Theory and Ligand Field Strength:
According to crystal field theory, the color of a coordination complex arises due to the splitting of the metal ion's d-orbitals in the ligand field. The energy difference between the split d-orbitals determines the wavelength of light absorbed by the complex. The stronger the ligand field, the greater the splitting of the d-orbitals, and this influences the color of the complex.

3. Ni(H₂O)₆²⁺ Complex:
In the case of Ni(H₂O)₆²⁺, water is a weak field ligand, and the crystal field splitting in this complex is relatively small. This results in the absorption of light in the visible spectrum, specifically in the red and yellow regions, leaving the complementary green color to be observed. The weak field strength of water leads to partially filled d-orbitals in the Ni²⁺ ion, which absorb light and produce the characteristic green color of the solution.

4. Ni(CN)₄²⁻ Complex:
In contrast, cyanide (CN⁻) is a strong field ligand. When CN⁻ coordinates to Ni²⁺, the crystal field splitting is large, which means the energy gap between the d-orbitals is significant. This results in the absorption of light at a much higher energy (in the ultraviolet region) and not in the visible spectrum. As a result, no visible light is absorbed, and the solution appears colorless. The strong ligand field of cyanide forces the d-electrons in the Ni²⁺ ion to pair up, which prevents any visible light absorption from occurring.

5. Final Answer:
A solution of Ni(H₂O)₆²⁺ is green because the weak field water ligands result in small crystal field splitting, allowing the absorption of visible light in the red and yellow regions. On the other hand, a solution of Ni(CN)₄²⁻ is colorless because the strong field cyanide ligands cause a large crystal field splitting, which leads to absorption of light in the ultraviolet region, leaving no visible absorption and thus no color.