The kinetics of the sequestration of iron(III) from the polynuclear iron(III) complex, [Fe11O6(OH)(6)(O2CPh)(15)]. H2O . 2MeCN, a possible biomimetic model of ferritin, with a series of 11 primary and secondary hydroxamic acids in dry acetonibrile/dichloromethane solvent mixtures indicates an associative interchange (Ia) mechanism in accordance with the wide variation of Delta H-not equal and Delta S-not subset of within in the series. Comparison of the kinetics of this hydroxamic acid series with the simple binuclear iron complex (NEt4)(2)[Fe2OCl6] shows that steric effects are considerably more important for the Fe-11 complex than for the Fe-2 complex. These conclusions, based on kinetics studies, are relevant to the design of chelators for sequestration of iron from ferritin.