Bovine heart cytochrome c oxidase and related heme copper oxidases are inhibited by cyanide, which binds at the binuclear heme-a(3)/Cu-B site where dioxygen is reduced to water. To determine the mode of cyanide binding, heme-based binuclear complexes containing iron-cyanide-copper bridges in different oxidation states have been prepared by the reaction of [(py)(OEP)Fc(CN)] with Cu(II,I) precursors and structurally characterized by X-ray methods. Structures of two precursor complexes and two binuclear Cu-I-CN-Cu-I species are reported. The assembly [(py)(OEP)Fe-CN-Cu(Npy(3))](2+) has a nearly linear Fe-III-CN-Cu-II bridge containing law-spin Fe(III). The assemblies [(OEP)Fe-NC-Cu(MeNpy(2))](+) and [(OEP-CH2CN)Fe-NC-Cu(Npy(3))](+) exhibit the highspin bridges Fe-III-NC-Cu-I and Fe-II-NC-Cu-I, respectively. These are the first title bridges in these oxidation states. Bridge atom sequences are obtained from structural refinements of both linkage isomers : those for the reduced bridges are consistent with the soft-acid nature of Cu(I). Cyanide stretching frequencies respond to metal oxidation state and bridge geometry and, using data for solution and solid states, fall into the following ranges : Fe-III-CN-Cu-II, 2120-2184 cm(-1) (11 examples); Fe-III-NC-Cu-I, 2072-2100 cm(-1) (2 examples) : Fe-II-NC-Cu-I, 2099-2107 cm(-1) (1 example). These data;cre compared with nu(CN) values for the enzymes in different oxidation states. A nonlinear Fe-III-CN-Cu-II bridge (Cu-N-C = 150-160 degrees) is consistent with the 2146-2152 cm(-1) range found for the fully oxidized enzymes. Bands that can be assigned with some certainty as Fe-CN vibrations in partially and fully reduced enzymes do not appear to correspond to Fe-III-NC-Cu-I and Fc(II)-NC-Cu-I bridges but rather to Fe-II-CN modes. The current work complements and extends our previous investigation (Scott and Holm, J. Am. Chem. Soc. 1994, 116, 11357) of linear and nonlinear Fe-III-CN-Cu-II bridges and is part of an investigation directed at providing a molecular basis of cyanide toxicity. (MeNpy(2) = bis(2-(2-pyridylethyl))-methylanline; Npy(3) = tris(2-pyridylmethyl)amine; OEP = octaethylpolphyrinate(2-), OEP-CH2CN = N-(cyanomethyl)octaethylporphyrinate(1-).).