The complex ion [Fe2III(μ-O)(phen) 4(H2O)2]4+ (1) (phen = 1,10-phenanthroline) and its hydrolytic derivatives [Fe2 III(μ-O)(phen)4(H2O)(OH)]3+ (1a) and [Fe2III(μ-O)(phen)4- (OH) 2]2+ (2a) coexist in rapid equilibria in the range pH 4.23-5.35 in the presence of excess phenanthroline (pKa1 = 3.71 ± 0.03, pKa2 = 5.28 ± 0.07). The solution reacts quantitatively with I- to produce [Fe(phen)3] 2+ and I2. Only 1 but none of its hydrolytic derivatives is kinetically active. Both inner and outer sphere pathways operate. The observed rate constants show second-order dependence on the concentration of iodide, while the dependence on [H+] is complex in nature. Added Cl- inhibits the formation of adduct with I- and thus retards the rate of inner sphere path, leading to a rate saturation at high [Cl-], where only the outer sphere mechanism is active. Kinetic data indicate that simultaneous presence of two l- in the vicinity of diiron core is necessary for the reduction of 1. © 2005 Wiley Periodicals, Inc.