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The correct order of the complexes [Co(NH3)5(H2O)]3+ (A), [Co(NH3)6]3+ (B), [Co(CN)6]3− (C) and [CoCl(NH3)5]2+ (D) in terms of wavelength of light absorbed is (1) C > B > A > D (2) D > C > B > A (3) C > B > D > A (4) D > A > B > C

1. C > B > A > D
2. D > C > B > A
3. C > B > D > A
4. D > A > B > C

Correct answer: D > A > B > C

Solution

The correct order reflects the increasing strength of the ligand field, which affects the energy gap between the d-orbitals and thus the wavelength of light absorbed. In this case, the weaker field ligands (like Cl-) lead to lower energy transitions, while stronger field ligands (like NH3 and CN-) result in higher energy transitions, causing the observed order.

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