Iron dissociation, kinetics

OsX2(dcpe)2]. This behavior parallels that for analogous ruthenium species. The cis complex may, depending on conditions, isomerize to the Irons form. Kinetic data are not available, but a dissociative mechanism is implied. 

The kinetics of iron(III) dissociation from a series of dihydrox-amate siderophores and siderophore mimics, including rhodo-torulic acid (3) and alcalagin, have been investigated (52,127,128, 177,178). ESI-MS studies show that these systems form multiple species as a function of pH and siderophore/iron ratio (128). The lability of these systems and the resultant multiple species leads to several parallel paths to iron(III) dissociation (177). Both the distribution of structures and kinetics of dissociation were shown to be dependent on the length of the spacer chain between the dihydroxamate donor groups (52,127). 

Activation volumes for aquation of Schiff base complexes [Fe(C5H4NCH=NHR)3]2+ (R = Me, Et, nPr, nBu) are between +11 and +14 cm3 mol-1 (107), and thus within the range established earlier (108) for (substituted) tris-l,10-phenanthroline-iron(II) complexes, viz. +11 to +22 cm3 mol-1. These positive values are consistent with dissociative activation. Kinetic studies of the reaction of a CH2S(CH2)3SCH2 -linked bis(terpy) ligand (L6) with [Fe(terpy)2]2+ showed a very slow two-step process. The suggested mechanism consisted of slow loss of one terpy, rapid formation of [Fe(terpy)(L6)], and finally slow displacement of the second terpy as the partially-bonded L6 becomes hexadentate (109). 

Fe(III) displacement of Al(III), Ga(III), or In(III) from their respective complexes with these tripodal ligands, have been determined. The M(III)-by-Fe(III) displacement processes are controlled by the ease of dissociation of Al(III), Ga(III), or In(III) Fe(III) may in turn be displaced from these complexes by edta (removal from the two non-equivalent sites gives rise to an appropriate kinetic pattern) (343). Kinetics and mechanism of a catalytic chloride ion effect on the dissociation of model siderophore-hydroxamate iron(III) complexes chloride and, to lesser extents, bromide and nitrate, catalyze ligand dissociation through transient coordination of the added anion to the iron (344). A catechol derivative of desferrioxamine has been found to remove iron from transferrin about 100 times faster than desferrioxamine itself it forms a significantly more stable product with Fe3+ (345). 

Kinetics and activation parameters for NO reactions with a series of iron(II) aminocarboxylato complexes have been obtained (Table II) in aqueous solution (31). Rate constants for these reactions ranged from 105 to 108M-1s-1 for the series of iron(II) complexes studied. The reactions of NO with Fen(edta) (edta = ethylenediaminetetraacetate) and Fen(Hedtra) (Hedtra = hydroxyethylenediaminetriacetate) yielded activation volumes of +4.1 and +2.8 cm3 mol-1, respectively and were assigned to a dissociative interchange (Id) mechanism (31b). All of the iron(II) aminocarboxylato complexes studied followed a similar pattern with the exception of the Fen(nta) (Nta = nitriloacetic acid) complex which gave a AV value of —1.5 cm3 mol-1. The reaction of this complex with... 

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