Mechanism of Iron(III) Complex Formation. Activation Volumes for the Complexation of the Iron(III) Ion with Thiocyanate Ion and Acetohydroxamic Acid

Activation volumes (ΔV^‡) for the complexation of the iron(III) ion with thiocyanate (SCN^-) and acetohydroxamic acid (Hahx) were determined by a high-pressure stopped-flow technique. The values of ΔV^‡ for reaction of Fe³⁺ are -6.1 - 1.0 cm³ mol^″1 for SCN^- and -10.0 - 1.4 cm³ mol^-1 for Hahx, whereas the ΔV‡ values of the FeOH²⁺ ion are 8.5 -1.2 cm³ mol^″1 for SCN^- and 7.7 - 0.6 cm³ mol^-1 for Hahx. Complexation of FeOH²⁺ is dissociatively activated but that of Fe associatively. No apparent parallel relationship is observed between ΔV‡ and corresponding AS values.