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The correct order of the following complexes in terms of their crystal field stabilization energies is (1) [Co(NH3)4]2+ < [Co(NH3)6]2+ < [Co(en)3]2+ (2) [Co(NH3)4]2+ < [Co(NH3)6]2+ < [Co(en)3]2+ (3) [Co(en)3]2+ < [Co(NH3)6]2+ < [Co(NH3)4]2+ (4) [Co(en)3]2+ < [Co(NH3)6]2+ < [Co(NH3)4]2+

1. (1)
2. (2)
3. (3)
4. (4)

Correct answer: (2)

Solution

The correct option is (2) because crystal field stabilization energy (CFSE) increases with the strength of the ligands and the number of ligands. In this case, ethylenediamine (en) is a stronger field ligand than ammonia (NH3), and the octahedral complex [Co(en)3]2+ has the highest CFSE, followed by [Co(NH3)6]2+, and [Co(NH3)4]2+ has the lowest due to fewer ligands.

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