The mononuclear [Cl-Fe-III(dpa)-Cl](+) (1(Cl)) complex containing a square planar tetradentate polypyridine ligand has been reported to catalyze water oxidation in pH = 1 aqueous medium with ceric ammonium nitrate (CAN) as a chemical oxidant. The reaction mechanism of the oxygen evolution driven by this catalyst was investigated by means of density functional calculations. The results showed that one chloride ligand of 1(Cl) has to exchange with a water molecule to generate 1, [Cl-Fe-III(dpa)-OH2](2+), as the starting species of the catalytic cycle. The initial one-electron oxidation of 1 is coupled with the release of two protons, generating [Cl-Fe-IV(dpa)=O](+) (2). Another one-electron transfer from 2 leads to the formation of an Fe-V=O complex [Cl-Fe-V(dpa)=O](2+) (3), which triggers the critical O-O bond formation. The electronic structure of 3 was found to be very similar to that of the high-valent heme-iron center of P450 enzymes, termed Compound I, in which a pi-cation radical ligand is believed to support a formal iron(IV)-oxo core. More importantly, 3 and Compound I share the same tendency toward electrophilic reactions. Two competing pathways were suggested for the O-O bond formation based on the present calculations. One is the nitrate nucleophilic attack on the iron(V)-oxo moiety with a total barrier of 12.3 kcal mol(-1). In this case, nitrate functions as a co-catalyst for the dioxygen formation. The other is the water nucleophilic attack on iron(V)-oxo with a greater barrier of 16.5 kcal mol(-1). In addition, ligand degradation via methyl hydrogen abstraction was found to have a barrier similar to that of the O-O bond formation, while the aromatic carbon hydroxylation has a higher barrier.