<P> The quest for well defined intermediates led to the characterization of many transition metal dinitrogen complexes . Few of these well defined complexes function catalytically, their behavior illuminated likely stages in nitrogen fixation . Most fruitful of all of these early studies focused on M (N) (dppe) (M = Mo, W). For example, double protonation of such low valent complexes gave intermediates with the linkage M = N − NH . In 1995, a molybdenum (III) amido complex was discovered that cleaved N to give the corresponding molybdenum (VI) nitride . This and related terminal nitrido complexes have been used to make nitriles . </P> <P> In 2003 a related molybdenum amido complex was found to catalyze the reduction of N. In addition to a source of protons, the catalyst requires a strong reducing agent . However, this catalytic reduction fixates only a few nitrogen molecules . In these systems, like the biological one, hydrogen is provided to the substrate heterolytically, by means of protons and reducing equivalents rather than with H itself . </P> <P> In 2011 Arashiba et al. reported yet another system with a catalyst again based on molybdenum but with a diphosphorus pincer ligand . Photolytic nitrogen splitting is also considered . </P>

Where does nitrogen go after plants use it