WATER EXCHANGE OF THE (ORTHO-PHENYLENEDIAMINE-N,N,N',N'-TETRAACETATO)FERRATE(III) COMPLEX IN AQUEOUS-SOLUTION AS STUDIED BY VARIABLE-TEMPERATURE, VARIABLE-PRESSURE, AND VARIABLE-FREQUENCY O-17 NMR TECHNIQUES

The crystal structure of potassium aqua(o-phenylenediamine-N,N,N',N'-tetraacetato)ferrate(III) dihydrate, (K[Fe(OH2)-(phdta)]-2H2O), has been determined by X-ray crystallography. The central iron(III) ion is seven-coordinate, with one water molecule being hydrated. Water-exchange rates of the iron(III) complex with phdta4- 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-degrees-C) = (1.2 +/- 0.2) x 10(7) s-1, DELTA-H-double-ended-dagger = 26 +/- 3 kJ mol-1, DELTA-S-double-ended-dagger = 22 +/- 9 J k-1 mol-1, and DELTA-V-double-ended-dagger = 4.6 +/- 0.2 cm3 mol-1. The positive activation volume indicates that the water exchange proceeds via a dissociative interchange mechanism.