Proton-driven ligand dissociation kinetics in the presence of chloride, bromide, and nitrate ions have been investigated for model siderophore complexes of Fe(III) with the mono- and dihydroxamic acid ligands R1C(=O)N(OH)R-2 (R-1 = CH3, R-2 = H; R-1 = CH3, R-2 = CH3; R-2 = C6H5, R-2 = H; R-1 = C6H5, R-2 = C6H5) and CH3N(OH)C(=O)[CH2](n)C(=O)N(OH)CH3 (H2Ln; n = 2, 4, 6). Significant rate acceleration in the presence of chloride ion is observed for ligand dissociation from the bis(hydroxamate)- and mono(hydroxamate)-bound complexes. Rate acceleration was also observed in the presence of bromide and nitrate ions but to a lesser extent. A mechanism for chloride ion catalysis of ligand dissociation is proposed which involves chloride ion dependent parallel paths with transient Cl-coordination to Fe(III). The labilizing effect of Cl- results in an increase in microscopic rate constants on the order of 10(2)-10(3). Second-order rate constants for the proton driven dissociation of dinuclear Fe(III) complexes formed with H2Ln were found to vary with Fe-Fe distance. An analysis of these data permits us to propose a reactive intermediate of the structure (H2O)(4)Fe(Ln)Fe(HLn)(Cl)(OH2)(2+) for the chloride ion dependent ligand dissociation path. Environmental and biological implications of chloride ion enhancement of Fe(III)-ligand dissociation reactions are presented.