Because of the pervasive occurrence of magnetically coupled siroheme and Fe4S4 units in assimilatory and dissimilatory sulfite and nitrite reductases, we have undertaken the synthesis of the sulfide-bridged assembly Fe4S4-S-heme as a possible analogue to the active sites of certain assimilatory enzymes. The approach has utilized iron subsite-differentiated clusters of the type [Fe4S4(LS(3))L’](2-), which undergo regiospecific substitution at the unique subsite. Reaction of [Fe4S4(LS(3))(SEt)](2-) with limited H2S in acetonitrile affords the functionalized cluster [Fe4S4(LS(3))(SH)](2-) (4), which exists in equilibrium with the mu-S double cubane {[Fe4S4(LS(3))]S-2}(4-) (6) and H2S. Reaction of 4 and [Fe(salen)]O-2 gave the bridged assembly [Fe4S4(LS(3))-S-Fe-III(salen)](2-) (8), detectable by its characteristic isotropically shifted H-1 NMR spectrum. Six routes were devised to a related heme-based assembly : directed acid-base coupling of 4 with [Fe(OEP)]O-2, [Fe(OEP)(OMe)], [Fe(OEP)(OC(Me)=CH2)], and [Fe(OEP)(OClO3)]/Et(3)N; Si-S bond cleavage in the reaction of [Fe4S4(LS(3))(SSiEt(3))](2-) with [Fe(OEP)F]; oxidative addition of [Fe-II(OEP)] to the disulfide bond of the mu-S-2 double cubane {[Fe4S4(LS(3))]S-2(2)}(4-) (7). In each case, the product was [Fe4S4(LS(3))-S-Fe-III(OEP)](2-) (9), recognizable by UV-visible absorption and H-1 NMR spectra. Both 8 and 9 contain [Fe4S4](2+) and high-spin Fe(III) fragments. Isotropic shifts mainly contact in origin that are enhanced by factors of 7-12 compared to those of precursor cluster 4, and the Curie-type temperature dependence of the shifts of 9, originate from extensive spin localization from the Fe(III) fragment to the cluster. This effect requires the existence of a covalent bridge between the fragments and, together with the methods of synthesis and other spectroscopic observations, provides structure proof of the bridged assemblies. These species sustain two one-electron reduction reactions; other reactions of 9, which alter or cleave the bridge, are summarized. The electronic features of bridged assemblies such as 8 and 9 approach the intrinsic magnetic and spectroscopic properties of a structurally similar unit in the oxidized enzymes and potentially provide a means of identification of such units. (LS(3) = 1,3,5-tris[(4,6-dimethyl-3-mercaptophenyl)-thio]-2,4,6-tris(p-tolylthio)benzene(3-); OEP octaethylporphyrinate(2-); salen = 1,2-bis(salicylidenearnino)ethane-(2-).)