Sulfur-donor ligands iron clusters with

Iron-iron bonds exist in carbonyl and nitrosyl derivatives, but these are not considered in this section. Properties consistent with Fe-Fe interactions in other systems are dominated by sulfur donor ligands. The presence of Fe-Fe bonds is not unambiguous, with sulfur bridges invariably accompanying short metal-metal separations. Synthetic routes to polynuclear iron-sulfur complexes are presented for completeness without implying the cluster bonding scheme. 

CuA-centers are found in cytochrome c oxidases and in N20-reductase [40,41]. In both enzyme classes, CuA-centers subtract electrons from an external donor and transfer them either directly to the active site or indirectly via a further redox-active center [42-44]. Until recently, knowledge concerning the structure of CuA-centers was incomplete. This situation was alleviated by the publication of the crystal-structures of cytochrome c oxidase from Paracoccus denitrificans and bovine heart in 1995 [43,44]. According to these data, CuA-centers contain [2Cu-2S] structures similar to those in [2Fe-2S]-type iron-sulfur clusters. Both sulfur ligands are donated by cysteine residues in the peptide chain and form a planar structure with the copper ions [43-45]. In both structures, an electron can be delocalized over both metal-ions. In the iron-sulfur center this effect is observed in the reduced form [FeZ5+-Fe2 5+], while in the CuA-center the delo-... 

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