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Consider the following complex ions, P, Q and R: P = [FeF₆]³⁻, Q = [Fe(H₂O)₆]³⁺, and R = [Fe(H₂O)₆]²⁺. The correct order of the complex ions according to their spin-only magnetic moment values (in B.M.) is:

1. R < Q < P
2. Q < R < P
3. R < P < Q
4. Q < P < R

Correct answer: Q < R < P

Solution

[Fe(H₂O)₆]³⁺ is high-spin d⁵ with 5 unpaired electrons, [Fe(H₂O)₆]²⁺ is high-spin d⁶ with 4 unpaired electrons, and [FeF₆]³⁻ is also high-spin d⁵ with 5 unpaired electrons. Since spin-only magnetic moment increases with the number of unpaired electrons, the order is Q < R < P as given in the options.

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