<P> Beginning in the 1960s several homogeneous systems were identified that convert nitrogen to ammonia, sometimes even catalytically but often operating via ill - defined mechanisms . The original discovery is described in an early review: </P> <P> "Vol'pin and co-workers, using a non-protic Lewis acid, aluminium tribromide, were able to demonstrate the truly catalytic effect of titanium by treating dinitrogen with a mixture of titanium tetrachloride, metallic aluminium, and aluminium tribromide at 50 ° C, either in the absence or in the presence of a solvent, e.g. benzene . As much as 200 mol of ammonia per mol of TiCl was obtained after hydrolysis. ..." </P> <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>

What type of organism is responsible for nitrogen fixation