In order to elucidate the structural and electronic properties of the dioxygen-reducing site of cytochrome oxidases, we have studied heme-based molecular assemblies containing the bridge unit Fe-III-X-Cu-II with X = O2-, OH-, and HCO2-. Here we describe the results of Mossbauer and magnetization studies of [(OEP)Fe-O-Cu(Me(6)-tren)](1+) (3), [(OEP)Fe-(OH)-Cu(Me(5)dien)(OClO3)](1+) (4), [(OEP)Fe-(O2CH)-Cu(Me(5)dien)(OClO3)](1+) (5), and [(OEP)Fe{(O2CH)Cu(Me(6)tren)}(2)](3+) (6). On the basis of magnetization studies, the oxidized binuclear enzyme site has been reported to be strongly coupled, with an S = 2 ground state. For 3 and 4 the Mossbauer data were analyzed for strong antiferromagnetic coupling between a high-spin Fe(III) (S-1 = 5/2) and Cu(II) (S-2 = 1/2), resulting in an S = 2 ground state. The exchange coupling of hydroxo-bridged 4 is substantial, J = 170 cm(-1) (H-ex = JS(1) . S-2) : but is Smaller than J greater than or equal to 200 cm(-1) reported for ore-bridged 3. The, collective evidence for synthetic complexes and the oxidized enzymes indicates that the Fe-III-(OH)-Cu-II bridge unit is probable for the latter in the resting state. The similarity of properties of formate-inhibited and "slow" cytochrome oxidase has suggested a carboxylate bridge in the binuclear site. Formate-bridged 5 maintains an anti-anti carboxylate bridge conformation and has J = +18 cm(-1). While the bridge conformation of 5 might not apply to a putative carboxylate bridge in the binuclear enzyme site, the order-of-magnitude difference between the J values of 3, 4, and 5 suggests that no stereochemically possible mu-eta(1):eta(1) carboxylate bridge conformation is likely to approach the strong coupling (J greater than or equal to 200 cm(-1)) of the formate-inhibited and "slow" enzyme forms. If carboxylate does bridge, the mu-eta(2) mode appears more likely. Complex 6, which is not biologically pertinent, displays weak ferromagnetic coupling between a central high-spin Fe(III) and two Cu(II) sites, -0.50 cm(-1) < J < -0.10 cm(-1).