A red solution formed by Fe^3+ and thiocyanate, [Fe(SCN)(H2O)5]^2+, is decolourised on adding excess sodium fluoride: [Fe(SCN)(H2O)5]^2+ + 6F- -> [FeF6]^3- + SCN- + 5H2O. The colourless complex formed and its spin-only magnetic moment (F- is a weak-field ligand) are

JEE Advanced levelSingle correct optionDifficulty 3 of 3qc198

A red solution formed by $Fe X^{3 +}$ and thiocyanate, $[Fe (SCN) (H X_{2} O) X_{5}] X^{2 +}$ , is decolourised on adding excess sodium fluoride: $[Fe (SCN) (H X_{2} O) X_{5}] X^{2 +} + 6 F X^{-} [FeF X_{6}] X^{3 -} + SCN X^{-} + 5 H X_{2} O$ . The colourless complex formed and its spin-only magnetic moment (F- is a weak-field ligand) are

Fluoride forms a more stable complex with Fe(III). Fe(III) is

d^{5}

; with weak-field

F X^{-}

it is high spin with 5 unpaired electrons,

μ = 5 \times 7 = 5.92 BM

. Charge:

+ 3 - 6 = - 3

. Low-spin (1 unpaired, 1.73 BM) applies to cyanide.