Three iron(III) complexes of N,N′-2-hydroxy-5-R-l,3-xylylenebis[N-(carboxymethyl)glycine] (5-R-HXTA; R = CH3, Cl) have been characterized in aqueous solution by visible, resonance Raman, NMR, and Mossbauer spectroscopy and magnetic susceptibility. The low-pH complex, [Fe2(5-Me-HXTA)OH(H2O)2], crystallizes in the space group C2/c (a = 21,074 (6), b = 10,260 (3), c = 23.931 (9) Å /? = 102,25 (3)°) and consists of two six-coordinate Fe(III) centers bridged by phenolate and hydroxide. The high-pH complex, (C4H10N)4[Fe4(5-Me-HXTA)2O2(OH)2], crystallizes in the space group P2/n (a = 14.465 (3), b = 1 1.379 (6), c = 20.530 (4) A; £ = 91.36 (2)°); the complex consists of a tetrahedron of iron(III) centers cumulatively bridged by six oxygens (two oxo, tw'o hydroxo, and two phenoxo groups), yielding an adamantane-like Fe406structure. These complexes retain their solid-state structures in solution on the basis of their spectroscopic properties. The intermediate-pH complex, characterized only in solution, is a tetranuclear complex derived from the dimerization of [Fe2-(5-Me-HXTA)0H(H20)2] by the replacement of a terminal aquo ligand on each binuclear complex by an oxo group bridging the two binuclear units. Such a structure is derived from the following observations: the concentration dependence of its extinction coefficient, the presence of an Fe-O-Fe vibration in its Raman spectrum, the Curie temperature dependence of some NMR features and the anti-Curie behavior of others indicative of the presence of both strongly coupled and weakly coupled pairs of iron(III) centers, and the appearance of two Mossbauer quadrupole doublets with large and smallaeq valuesin a 1:1 ratio. The solution behavior of the HXTA complex is contrasted to that of the corresponding N-(o-hydroxybenzyl)-N-(carboxy-methyl)glycine complexes, which retain their mononuclear character over the pH range studied. This difference in oligomerization tendency is attributed to the difference in Lewis acidity of the iron(III) centers.