Question

Why does aqueous Fe(III) ion, develop intense red colour when it reacts with $SC{N}^{-}$ ion while Fe(II) ion does not?

• A. Fe(III) ion forms a charge transfer complex with $SC{N}^{-}$ ions
• B. Fe(III0 is reduced to Fe(I) which is deep red in colour
• C. $SN{C}^{-}$ ion is oxidised to $C{N}^{-}$ which forms red coloured complex with Fe(III) ion
• D. $SC{N}^{-}$ does not form any complex with Fe(III) ion

Answer 1

The correct option is A Fe(III) ion forms a charge transfer complex with $SC{N}^{-}$ ions

$F{e}^{3+}$ reacts with thiocyanate to give a series of intensely red colored compounds which remain in the solution and does not undergo any reaction after the formation of the stable octahedral structure. In high concentration of $SC{N}^{-}$ the compound is fairly octahedral with six thiocyanate ligands bonded to iron atom.
Colour formation is due to unpaired electron and formation of charge transfer complex.