Progress toward the development of functional models of the carboxylate-bridged diiron active site in soluble methane monooxygenase is described in which potential substrates are introduced as substituents on bound pyridine ligands. Pyridine ligands incorporating a thiol, sulfide, sulfoxide, or phosphine moiety were allowed to react with the preassembled diiron(II) complex [Fe-2(mu-O2CArR)(2)(O2CArR)(2)(THF)(2)], where (-O2CArR) is a sterically hindered 2,6-di(p-tolyl)- or 2,6-di(p-fluorophenyl)benzoate (R = Tol or 4-FPh). The resulting diiron(II) complexes were characterized crystallographically. Triply and doubly bridged compounds [Fe-2(mu-O2CArTol)(3)(O2CArTol)(2-MeSpy)] (4) and [Fe-2(mu-O2CArTol)(2)(O2CArTol)(2)(2-MeS(O)py)(2)] (5) resulted when 2-methylthiopyridine (2-MeSpy) and 2-pyridylmethyl-sulfoxide (2-MeS(O)py), respectively, were employed. Another triply bridged diiron(II) complex, [Fe-2(mu-O2CAr4-FPh)(3)-(O(2)CAO(4-FPh)) (2-Ph(2)Ppy)] (3), was obtained containing 2-diphenylphosphinopyridine (2-Ph2PPY), The use of 2-mercaptopyridine (2-HSpy) produced the mononuclear complex [Fe(O2CArTol)(2)(2-HSPY)(2)] (6a). Together with that of previously reported [Fe-2(mu-O2CArTol)(3)(O2CArTol)(2-PhSpy)] (2) and [Fe-2(mu-O2CArTol), (O2CArTol)(2-Ph2PPY)] (1), the dioxygen reactivity of these iron(II) complexes was investigated. A dioxygen-dependent intermediate (6b) formed upon exposure of 6a to 02, the electronic structure of which was probed by various spectroscopic methods. Exposure of 4 and 5 to dioxygen revealed both sulfide and sulfoxide oxidation. Oxidation of 3 in CH2Cl2 yields [Fe-2(mu-OH)(2)-(mu-O2CAr4-FPh) (O2CAr4-FPh)(3)(OH2)(2-Ph2P(O)py)] (8), which contains the biologically relevant {Fe-2(mu-OH)(2)-(mu-O2CR)}(3+) core. This reaction is sensitive to the choice of carboxylate ligands, however, since the p-tolyl analogue 1 yielded a hexanuclear species, 7, upon oxidation.