The crystal structure of potassium aqua(o-phenylenediamine-N,N,"'-tetraacetato)ferrate(III) dihydrate, (K[Fe(OH2)-(phdta)].2H20), has been determined by X-ray crystallography. The central iron(II1) ion is seven-coordinate, with one water molecule being hydrated. Water-exchange rates of the iron(II1) complex with phdtaC in aqueous solution have been studied as a function of temperature and pressure by the oxygen-17 NMR line-broadening method. Activation parameters for water exchange have been determined as follows: k(25 'C) = (1.2 i 0.2) X IO' s-I, AH* = 26 i 3 kJ mol-', AS* = -22 * 9 J K-' mol-', and AI" = 4.6 f 0.2 cm3 mol-'. The positive activation volume indicates that the water exchange proceeds via a dissociative interchange mechanism.
IntroductionValuable results have been accumulated from a number of studies on t h e solvent exchange a t solvated metal ions by the high-pressure NMR method. For example, the study of water exchange on divalent metal ions across the first-row transitionmetal series has demonstrated the changeover from associative interchange I, to dissociative interchange Id mechanisms along the series.Is2 This changeover has been rationalized in terms of changes in ionic radius and of filling of d orbitals. The solvent exchange of the d5 manganese(I1) ion is known to proceed via an associative interchange mechanism in water,' methan01,~ and a~etonitrile.~ Recently, however, positive volumes of activation have been reported for solvent exchange on t h e manganese( 11) ion in N,N-dimethylformamide (DMF)S and acetic acid.6 Such a positive activation volume has been interpreted in terms of the bulkiness of coordinated solvent molecules. Furthermore, the activation volume for the solvent exchange of manganese(I1) acetate in acetic acid has been determined t o be +6.7 f 0.6 cm3 which is more positive than that (+0.4 f 0.7 cm3 mol-') for the hexakis(solvent)manganese(II) ion in acetic acid. I t has been claimed that the coordinated solvent molecules are labilized by electron donation from bound ligands such as CH3COO-t o the metal ion and that the activation mode of the reaction becomes less associative.
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