Mechanistic Difference of the Iron(III) Ion Complexation in Different Solvents

The activation volumes (ΔK^‡) for the complexation of the iron(III) ion with acetohydroxamic acid, 4-isopropyltropolone, and thiocyanate ion in N,N-dimethylformamide (DMF) and dimethyl sulfoxide (Me₂SO) were determined by a high-pressure stopped-flow technique. Values of ΔV^‡) for the formation of the (acetohydroxamato)iron(III) and (isopropyltropolonato)iron(III) complexes are -0.8 ± 0.2 and 5.0 ± 0.4 cm³ mol^-1, respectively, in DMF and 3.0 ± 0.3 and 10.9 ± 1.7 cm³ mol^-1 in Me₂SO, respectively. For the formation and dissociation of the 1:1 (isothiocyanato)iron(III) complex in Me₂SO, the ΔV+ values are 3.3 ± 0.6 and -8.5 ± 1.4 cm³ mol^-1, respectively. The change in the sign of ΔV^‡) is interpreted on the basis of relative bulkiness of both the entering ligand and the coordinated solvent molecule.