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When [Fe(CN)6]³- is converted to [FeF6]³-, which of the following changes is observed?

1. Decrease in magnetic moment
2. Increase in magnetic moment
3. Decrease in coordination number
4. Increase in coordination number

Correct answer: Increase in magnetic moment

Solution

In [Fe(CN)6]³-, CN- is a strong-field ligand giving low-spin Fe³+ (d⁵) with 1 unpaired electron and mu = sqrt(3) BM approx 1.73 BM. In [FeF6]³-, F- is a weak-field ligand giving high-spin Fe³+ with 5 unpaired electrons and mu = sqrt(35) BM approx 5.92 BM. Therefore the magnetic moment increases, while the coordination number stays 6 in both.

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