In our efforts to model the oxygen activation chemistry of methane monooxygenase (MMO) and the R2 protein of ribonucleotide reductase (RNR), we have discovered a transient green species (3) in the reaction of H2O2 with a (mu-oxo)diiron(III) TPA complex (TPA = tris(2-pyridylmethyl)amine). Our studies show that the precursor to 3 is [Fe2O(TPA)(2)(OH)(H2O)](ClO4)(3) (2a), which can be obtained by the treatment of [Fe2O(TPA)(2)(H2O)(ClO4)]-(ClO4)(3) (1) With an equivalent of base. Crystallographic studies show that 1 has a nearly linear (mu-oxo)diiron(III) core with terminal aqua and perchlorato ligands (angle Fe-(mu-O)-Fe = 174.1(4)degrees), while 2c, the 5-Et-TPA analogue of 2a, has a bent (mu-oxo)diiron(III) core that is supported by an H3O2- bridge, The presence of an H3O2- bridge in the latter is indicated by the short O-O separation (2.464(9) Angstrom), the Fe-Fe distance of 3.346(9) Angstrom, and the Fe-(mu-O)-Fe angle of 136.3(3)degrees. Thus treatment of 1 with an equivalent of base results in the replacement of the bound perchlorate with hydroxide and the bending of the Fe-O-Fe unit to form 2, That the bent Fe-O-Fe core persists in solution is indicated by its UV-vis features and NMR spectra that reflect distinct TPA coordination modes about the individual iron sites. The green intermediate 3 is generated by the reaction of 2, [Fe2O(L)(2)(OH)(H2O)](ClO4)(3) (L = TPA, 5-Me-TPA, and 5-Et-TPA), with H2O2 in CH3CN at -40 degrees C; when 5-Me-TPA is used as the tripodal ligand, 3b can be isolated as a solid upon standing overnight at -40 degrees C.