Letter | Published:

Transition Metal Complexes of Diazonium Salts as Models for Nitrogenase


THE recent synthesis of a wide variety of transition metal complexes of molecular nitrogen (dinitrogen) has underlined the strong possibility that formation of a molybdenum—or iron—dinitrogen complex is an initial step in biological nitrogen fixation. Furthermore, complexes have been synthesized in which the dinitrogen ligand bridges two transition metals. These exhibit a substantial reduction in the N–N stretching frequency, ν(N2), and it may well be that dinitrogen is activated by bridging iron and molybdenum in the metallo-enzyme itself1. Specifically, Chatt has suggested that dinitrogen is probably taken up initially at the iron site, to form a dinitrogen complex of rather low ν(N2), which is lowered still further by the attachment of the molybdenum to the free end of the dinitrogen ligand, and thereby made more susceptible to facile reduction. An extension of this hypothesis is that of R. W. F. Hardy and E. Knight who suggest (personal communication) that one metal may be present as a hydride, in which case the following intermediate would result from insertion of the dinitrogen complex into the metal hydride bond:

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