Ozutsumi K, Kurihara M, Kawashima T
Laboratory of Analytical Chemistry, Department of Chemistry, University of Tsukuba, Tsukuba 305, Japan.
Talanta. 1993 May;40(5):599-607. doi: 10.1016/0039-9140(93)80264-r.
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(2)H(5))(4)NClO(4) or 1 mol/dm(3) NH(4)ClO(4) as a constant ionic medium at 25 degrees C. Calorimetric titration data were well explained in terms of the formation of Fe(SCN)(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. DeltaG degrees (1) < DeltaG degrees (2) < DeltaG degrees (3) < DeltaG degrees (4), < DeltaG degrees (5), DeltaH degrees (1) > DeltaH degrees (2) > DeltaH degrees (3) > DeltaH degrees (4) > DeltaH degrees (5), DeltaS degrees (1) > DeltaS degrees (2) > DeltaS degrees (3) > DeltaS degrees (4) > DeltaS degrees (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) (n = 1-5) complexes have a six-coordinate octahedral structure as well as the Fe(dmf)(6) ion, the octahedral structure of which has been confirmed by the EXAFS (extended X-ray absorption fine structure) method.
