A series of molecular complexes containing the [Fe-III-CN-Cu-II] bridge unit has been prepared and structurally characterized as a means of establishing the presence of a similar bridge in cyanide-inhibited cytochrome c oxidase and terminal quinol oxidases. The series consists of the molecules [(py)(OEP)Fe-CN- Cu(L/L’)](2+/+) with L/L’ = Me(5)dien/Me(2)CO (11) and bnpy(2)/OTf (12), and L = TIM (13), cyclops (14), and cyclam (15). These species were prepared by the reaction of [Fe(OEP)(py)(CN)] (1) with the appropriate Cu(II) precursors in acetone or acetonitrile/dichloromethane solutions. Because Cu-B sites in enzymes are structurally undefined, the stereochemistry of these precursors was varied. When combined with the previously reported species L = Me(6)tren (9) and L/L’ = Me(5)dien/OTf (10), they constitute a set of seven monobridged species. Also prepared was {[(py)(OEP)Fe(CN)]Cu-2-(cyclam)}(2+) (16), shown to contain the [Fe-III-CN-Cu-II-NC-Fe-III] bridge, and [(OEP)Fe-NC-Cu(Me(5)dien)](+) (17). The X-ray structures of precursor complexes [Cu(Me(5)dien)(OH2)(OCMe(2))](2+), [Cu(bnpy(2))(OTf)(2)], [Cu(cy-clam)(MeCN)(2)](2+), and [Cu(Me(5)dien)(MeCN)](+) and the bridged assemblies 11-14, 16, and 17 were determined; those of 9 and 10 have been described earlier. The structures of the heme and Cu(II) fragments differ little from those of 1 and the Cu(II) starting compounds, respectively. In all bridges, the Fe-III-CN fragment is linear (bond angle 176-180 degrees) and any deviations therefrom occur in the Cu-II-CN fragment. Nearly linear bridges occur in 9 (trigonal bipyramidal-ax) and 10/11 (distorted square pyramidal-eq) (Cu-N-C = 170-174 degrees). Bent bridges are found in 12 (163 degrees, distorted square pyramidal-eq). and in 13 (147 degrees) and 14 (159 degrees) (both square pyramidal-ax). The smallest angle is observed in doubly bridged 16 (140 degrees, tetragonal octahedral). Ln the bridges, decreasing Cu-N-C bond angles correlate with increasing Cu-NC bond distances. Cyanide stretching frequencies in the crystalline state and in dichloromethane solution are nearly the same and in the solids cover the interval 2181-2120 cm(-1); nearly all values exceed that of 1 (2129 cm(-1)). Increasing nu(CN) values correlate with decreasing Cu-N-C angles and increasing Cu-N bond distances. These values encompass those for cyanide-inhibited oxidized enzymes (2152-2146 cm(-1)). The infrared data, together with magnetic and Mossbauer spectroscopic results which show coupled Fe(III) and Cu(II) spins, lead to the conclusion that these enzymes possess a tight [Fe-III-CN-(CuI)-I-I] bridge, rather than a binuclear site in which cyanide is bridged exclusively to one subsite, or interacts weakly with another subsite, or is hydrogen-bonded at the uncoordinated end. The existence of the [Fe-III-NC-Cu-I] bridge in 17 raises the matter of a similar atom sequence in biological bridges of the same oxidation state. (bnpy(2) = N,N-bis(2-(2-pyridylethyl))benzylamine, Me(5)dien = 1,1,4,7,7-pentamethyldiethylenetriamine, Me(6)tren = tris(2-(dimethylamino)ethyl)amine, OEP = octaethylporphyrinate(2-), OTf = CF3SO3-, py = pyridine; cyclops, TIM, cyclam = tetraazamacrocycles; ax = axial, eq = equatorial, referring to the position of cyanide substitution in the Cu(II) coordination sphere.)