STRUCTURE OF IRON(III) ION AND ITS COMPLEXATION WITH THIOCYANATE ION IN N,N-DIMETHYLFORMAMIDE

Complexation of iron(III) with thiocyanate ions has been calorimetrically and spectrophotometrically investigated in N,N-dimethylformamide (DMF) containing 0.4 mol/dm3 (C2H5)4NClO4 or 1 mol/dm3 NH4ClO4 as a constant ionic medium at 25-degrees-C. Calorimetric titration data were well explained in terms of the formation of [Fe(SCN)n](3-n)+ (n = 1-5), and their formation constants, reaction enthalpies and entropies were determined. Electronic spectra of individual iron(III) thiocyanato complexes were also determined. The stepwise thermodynamic quantities changed monotonously, i.e. DELTAG1-degrees < DELTAG2-degrees < DELTAG3-degrees < DELTAG4-degrees < DELTAG5-degrees, DELTAH1-degrees > DELTAH2-degrees > DELTAH3-degrees > DELTAH4-degrees > DELTAH5-degrees, DELTAS1-degrees > DELTAS2-degrees > DELTAS3-degrees > DELTAS4-degrees > DELTAS5-degrees. This suggests that no extensive desolvation occurred at any step of complexation. On the basis of these thermodynamic quantities, it is postulated that the [Fe(SCN)n](3-n)+ (n - 1-5) complexes have a six-coordinate octahedral structure as well as the [Fe(dmf)6]3 ion, the octahedral structure of which has been confirmed by the EXAFS (extended X-ray absorption fine structure) method.