Question

If a didentate ligand ethane -1,2- diamine is progressively added in the molar ratio en : Ni :: 1 : 1, 2 : 1, 3 : 1 to [Ni(H₂O)₆]²⁺ aq solution, following co-ordination entities are formed.
I. [Ni(H₂O)₄en]²⁺_(aq) - pale blue
II. [Ni(H₂O)₂(en)₂]²⁺_(aq) - blue/purple
III. [Ni(en)₃]²⁺_(aq) - violet
The wavelength in nm of light absorbed in case of I and III are respectively

• A. 475 nm and 310 nm
• B. 300 nm and 475 nm
• C. 310 nm and 500 nm
• D. 600 nm and 535 nm

Answer 1

The Correct Option is D

In this case, we are considering the coordination complex formation involving the Ni²⁺ ion and ethane-1,2-diamine (en), which is a bidentate ligand. The Ni²⁺ ion, initially present in the form of [Ni(H₂O)₆]²⁺, forms different coordination complexes as more en ligands are added.

Complex Formation:

[Ni(H₂O)₄en]²⁺: When one en ligand is added in a 1:1 ratio with Ni²⁺, this forms the complex [Ni(H₂O)₄en]²⁺. This complex absorbs light at a wavelength of 600 nm and appears pale blue in color.

[Ni(H₂O)₂(en)₂]²⁺: When en is added in a 2:1 ratio with Ni²⁺, a second en ligand coordinates with the Ni²⁺ ion, forming [Ni(H₂O)₂(en)₂]²⁺. This complex absorbs light at a wavelength of approximately 600 nm as well, and it appears blue/purple in color.

[Ni(en)₃]²⁺: When en is added in a 3:1 ratio with Ni²⁺, the Ni²⁺ ion forms a complex with three en ligands, resulting in [Ni(en)₃]²⁺. This complex absorbs light at a wavelength of 535 nm and appears violet in color.

Absorption Wavelengths:

For complex I ([Ni(H₂O)₄en]²⁺), the absorbed wavelength is 600 nm.

For complex III ([Ni(en)₃]²⁺), the absorbed wavelength is 535 nm.

Conclusion:

The wavelengths of light absorbed for complexes I and III are:

600 nm for [Ni(H₂O)₄en]²⁺ (pale blue).

535 nm for [Ni(en)₃]²⁺ (violet).

Thus, the answer is 600 nm and 535 nm.