The composition of a (μ-oxo)diiron(III)complex coordinated by tris[(3,5-dimethyl-4-methoxy)pyridyl-2-methyl]amine (R 3TPA)ligands was investigated. Characterization using a variety of spectroscopic methods and X-ray crystallography indicated that the reaction of iron(III)perchlorate, sodium hydroxide, and R 3TPA affords [Fe 2(μ-O)(μ-OH) (R 3TPA) 2](ClO 4) 3 (2)rather than the previously reported species [Fe 2(μ-O)(OH)(H 2O)(R 3TPA) 2](ClO 4) 3 (1). Facile conversion of the (μ-oxo)(μ-hydroxo)diiron(III)core of 2 to the (μ-oxo)(hydroxo)(aqua)diiron(III)core of 1 occurs in the presence of water and at low temperature. When 2 is exposed to wet acetonitrile at room temperature, the CH 3CN adduct is hydrolyzed to CH 3COO-, which forms the compound [Fe 2(μ-O)(μ-CH 3COO)(R 3TPA) 2](ClO 4) 3 (10). The identity of 10 was confirmed by comparison of its spectroscopic properties with those of an independently prepared sample. To evaluate whether or not 1 and 2 are capable of generating the diiron(IV)species [Fe 2(μ-O)(OH)(O)(R 3TPA) 2] 3+ (4), which has previously been generated as a synthetic model for high-valent diiron protein oxygenated intermediates, studies were performed to investigate their reactivity with hydrogen peroxide. Because 2 reacts rapidly with hydrogen peroxide in CH 3CN but not in CH 3CN/H 2O, conditions that favor conversion to 1, complex 1 is not a likely precursor to 4. Compound 4 also forms in the reaction of 2 with H 2O 2 in solvents lacking a nitrile, suggesting that hydrolysis of CH 3CN is not involved in the H 2O 2 activation reaction. These findings shed light on the formation of several diiron complexes of electron-rich R 3TPA ligands and elaborate on conditions required to generate synthetic models of diiron(IV)protein intermediates with this ligand framework.