COMPLEXATION OF IRON(III) WITH THIOCYANATE IONS IN AQUEOUS-SOLUTION

Spectrophotometric and calorimetric studies of the complexation of iron(III) with thiocyanate ions have been performed in an aqueous solution containing 1 mol dm-3 NaClO4 or NH4ClO4 as a constant ionic medium at 25-degrees-C. Spectrophotometric titration data were well explained in terms of the formation of [Fe(SCN)]2+, [Fe(SCN)2]+ and [Fe(SCN)3], and their formation constants and individual electronic spectra were determined. The [Fe(SCN)]2+, [Fe(SCN)2]+ and [Fe(SCN)3] complexes exhibit absorption maxima at 450 - 500 nm. Enthalpies and entropies for the formation of the [Fe(SCN)]2+, [Fe(SCN)2]+ and [Fe(SCN)3] complexes were also determined from calorimetric titration data. On the basis of these thermodynamic quantities, it is postulated that the [Fe(SCN)]2+, [Fe(SCN)2]+ and [Fe(SCN)3] complexes have a six-coordinate octahedral structure.