Cover Picture (Angew. Chem. Int. Ed. Engl. 9/1989)


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The cover illustration shows a space-filling model of a complex that may contribute to the understanding of biological nitrogen fixation. This process is catalyzed by nitrogenases, in all of which the dominant metal is iron surrounded by a coordination sphere of sulfur. Most nitrogenases also contain molybdenum, and some include vanadium. Recently, however, another type of nitrogenase has been characterized that contains only iron. In the model complex shown here, the extremely unstable molecule diazene (HN[DOUBLE BOND]NH) is bound to two iron atoms, each of which is pseudo-octahedrally coordinated to five donor atoms: four sulfurs and an amine nitrogen. Diazene is the first two-electron reduction product of N2. In addition to complexation and considerable steric blocking, diazene is here stabilized by powerful hydrogen bonding. More information regarding this bioinorganic model compound is presented by D. Sellmann et al. on p.1271 ff. (The current issue also contains other important contributions relevant to this subject).