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JEE Advanced levelSingle correct optionDifficulty 3 of 3cc097

An octahedral complex $K X_{x} [FeF X_{6}]$ has iron in the +2 or +3 state. F- is a weak-field ligand and the spin-only moment is 4.90 BM. The oxidation state of Fe and its CFSE ignoring pairing energy are

A+3 and $- 2.0 Δ_{o}$
B+3 and $0$
C+2 and $- 0.4 Δ_{o}$
D+2 and $- 2.4 Δ_{o}$

Answer: C

4.90 BM needs 4 unpaired electrons: high-spin

d^{6}

gives 4, while high-spin

d^{5}

gives 5 (

μ = 5.92

). So Fe is +2 and

x = 4

. CFSE of

t_{2 g}^{4} e_{g}^{2}

is

4 (- 0.4) + 2 (0.6) = - 0.4 Δ_{o}

.

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