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Consider the following complexes. [CoCl(NH3)5]2+ (A) [Co(CN)6]3− (B) [Co(NH3)5(H2O)]3+ (C) [Cu(H2O)4]2+ (D) The correct order of A, B, C and D in terms of wavenumber of light absorbed is :

1. (1) C < D < A < B
2. (2) D < A < C < B
3. (3) A < C < B < D
4. (4) B < C < A < D

Correct answer: (2) D < A < C < B

Solution

The order of light absorption wavenumbers is determined by the strength of the ligand field around the metal ions. In this case, water is a weak field ligand, leading to the lowest energy transition in D, while CN⁻ is a strong field ligand in B, resulting in the highest energy transition. Thus, the correct order reflects the increasing strength of the ligands from D to B.

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