Structure of iron(III) ion and its complexation with thiocyanate ion in N, N-dimethylformamide

Abstract

Complexation of iron(III) with thiocyanate ions has been calorimetrically and spectrophotometrically investigated in N, N-dimethylformamide (DMF) containing 0.4 mol/dm 3 (C 2H 5) 4NClO 4 or 1 mol/dm 3 NH 4ClO 4 as a constant ionic medium at 25°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. Δ G° 1 < Δ G° 2 < Δ G° 3 < Δ G° 4, < Δ G° 5, Δ H° 1 > Δ H° 2 > Δ H° 3 > Δ H° 4 > Δ H° 5, Δ S° 1 > Δ S° 2 > Δ S° 3 > Δ S° 4 > Δ S° 5. 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.