Nitrogen-Fixing Reactions

Briefly, the dinitrogen complexes listed in Table 1 are prepared by a number of different methods. The most common method involves the reduction in an ether solvent under a dinitrogen atmosphere of either a phosphine complex of a metal halide or a mixture of metal salt and appropriate phosphine. The only other frequently used method is ligand displacement by dinitrogen. To illustrate these methods, a number of high-yield preparations of -Mo(N2)2(dppe)2 are given in Equations 

In the following sections, nitrogen-fixing reactions will be reviewed element by element rather than according to the type of reaction. All reactions are reductive. There are no examples to date of oxidative nitrogen fixation within the confines of this chapter. 

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Not all chemical reactions proceed at rates fast enough to create significant deviations from NAECs. For example, although N2 gas is consumed by nitrogen-fixing plankton and produced by denitrifying bacteria, these processes are too slow to affect the relatively high seawater concentrations established by equilibration with the atmosphere. 

Also included in Table 7.7 are the nitrogen fixation reactions. These are similar to the carbon fixation reactions in that they involve the conversion of an oxidized inorganic species (N2) 1° a reduced form, such as ammonium. The fixed forms of nitrogen can be taken up by plants. As with carbon fixation, this process requires an energy source in order to proceed. Some N2 fixers are photosynthetic and others use energy obtained from the oxidation of reduced inorganic compounds. 

Bacterial ferredoxins function primarily as electron carriers in ferredoxin-mediated oxidation reduction reactions. Some examples are reduction of NAD, NADP, FMN, FAD, sulfite and protons in anaerobic bacteria, CO -fixation cycles in photosynthetic bacteria, nitrogen fixation in anaerobic nitrogen fixing bacteria, and reductive carboxylation of substrates in fermentative bacteria. The roles of bacterial ferredoxins in these reactions have been summarized by Orme-Johnson (2), Buchanan and Arnon (3), and Mortenson and Nakos (31). 

The effect of pH on K for the nitrogen fixation reaction (9.5-2) is so striking (a change of 1010 per pH unit) that it is shown in Fig. 9.3. Note that nitrogen cannot be fixed by this reaction above pH 8 when ferredoxin from Claustridium pasteurianum is used. 

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