Methane mono-oxygenase (MMO) and deoxyhemerythrin (DHr) are examples of di-iron enzymes that catalyze the dissociative and non-dissociative binding of molecular oxygen. To mimic the MMO active site with a finite cluster, we chose to study the binuclear heptapodate coordinated iron(III)-complexes of N,N,N,N'-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diamino-propane (HPTB) and N,N,N',N'-tetrakis(2-pyridylmethyl)-2-hydroxy-1,3-diamino-propane (HPTP). These have active sites of the form [Fez (HPTP)(mu -OH)](4+) (1) and [Fe-2(HPTB)(mu -OH)](4+) (2). Quantum mechanics structures are compared with the experimental data obtained from the EXAFS analysis. For the O-2 binding on the reduced active site. the mu-eta (1):eta (1)-O-2 mode seems the slightly more stable precursor to the O=Fe-O-Fe=O bis-ferryl (re)active site. The nature of the ferryl groups are these of a reactive two center three electron bond. (C) 2001 Elsevier Science B.V. All rights reserved.