| Abstract
| - The iron(III) hydroxo dimer, Fe2(μ-OH)2(H2O)84+, forms relatively stable dinuclear complexes with sulfate, hypophosphite, phosphite, arsenite, and selenite ions and a tetranuclear complex with arsenate ion. These transient species undergo decomposition into the thermodynamically stable mononuclear complex of the corresponding ligand. The kinetics of the reactions can be interpreted on the basis of the scheme shown here.
- The kinetics and mechanisms of ligand substitution reactions of the iron(III) hydroxo dimer, Fe2(μ-OH)2(H2O)84+,with various inorganic ligands were studied by the stopped-flow method at 10.0 or 25.0 °C in 1.0 M NaClO4. Thetransient formation of the following di- and tetranuclear complexes was confirmed: Fe2(OH)SO43+, Fe2(OH)H2PO24+, Fe2(OH)HPO33+, Fe2(OH)SeO33+, and Fe4(AsO4)(OH)27+. The catalytic effect of arsenic(III) on the hydrolyticreaction of iron(III) was also attributed to the formation of a dinuclear complex at very low concentration levels.Fast formation and subsequent dissociation of the multinuclear species into the corresponding mononuclear complexes(FeL) proceed via parallel reaction paths which, in general, show composite pH dependencies. The appropriaterate laws were established. The reactions of the different ligands occur at very similar rates, though the uninegativelycharged singly deprotonated form reacts about 1 order of magnitude faster than the neutral form of the sameligand. The results can conveniently be interpreted in terms of a dissociative interchange mechanism which postulatesthe formation of an intermediate complex in which the ligand is coordinated to only one Fe(III) center of the hydroxodimer. In a subsequent fast step, the ligand forms a bridge between the two metal ions by replacing one of the OHgroups. The dissociation of the dinuclear complex into FeL most likely proceeds via the same intermediate.
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