Editing Nitrogen (section)

===Dinitrogen complexes===
{{main|Dinitrogen complex}}
[[File:RuA5N2.png|thumb|right|Structure of [Ru(NH<sub>3</sub>)<sub>5</sub>(N<sub>2</sub>)]<sup>2+</sup> ([[pentaamine(dinitrogen)ruthenium(II) chloride|pentaamine(dinitrogen)ruthenium(II)]]), the first dinitrogen complex to be discovered]]
The first example of a [[dinitrogen complex]] to be discovered was [Ru(NH<sub>3</sub>)<sub>5</sub>(N<sub>2</sub>)]<sup>2+</sup> (see figure at right), and soon many other such complexes were discovered. These [[Complex (chemistry)|complexes]], in which a nitrogen molecule donates at least one lone pair of electrons to a central metal cation, illustrate how N<sub>2</sub> might bind to the metal(s) in [[nitrogenase]] and the [[Catalysis|catalyst]] for the [[Haber process]]: these processes involving dinitrogen activation are vitally important in biology and in the production of fertilisers.<ref>{{Cite journal|author=Fryzuk, M. D.|author2=Johnson, S. A.|name-list-style=amp|title=The continuing story of dinitrogen activation|journal=Coordination Chemistry Reviews|volume=200–202|page=379|year=2000|doi=10.1016/S0010-8545(00)00264-2}}</ref><ref name="Schrock">{{Cite journal|author=Schrock, R. R.|title=Catalytic Reduction of Dinitrogen to Ammonia at a Single Molybdenum Center|journal=[[Acc. Chem. Res.]]|year=2005|volume=38|issue=12|pmid=16359167|pages=955–62|pmc=2551323|doi=10.1021/ar0501121|author-link=Richard R. Schrock}}</ref>

Dinitrogen is able to coordinate to metals in five different ways. The more well-characterised ways are the end-on M←N≡N (''[[hapticity|η]]''<sup>1</sup>) and M←N≡N→M (''[[bridging ligand|μ]]'', bis-''η''<sup>1</sup>), in which the lone pairs on the nitrogen atoms are donated to the metal cation. The less well-characterised ways involve dinitrogen donating electron pairs from the triple bond, either as a [[bridging ligand]] to two metal cations (''μ'', bis-''η''<sup>2</sup>) or to just one (''η''<sup>2</sup>). The fifth and unique method involves triple-coordination as a bridging ligand, donating all three electron pairs from the triple bond (''μ''<sub>3</sub>-N<sub>2</sub>). A few complexes feature multiple N<sub>2</sub> ligands and some feature N<sub>2</sub> bonded in multiple ways. Since N<sub>2</sub> is isoelectronic with [[carbon monoxide]] (CO) and [[acetylene]] (C<sub>2</sub>H<sub>2</sub>), the bonding in dinitrogen complexes is closely allied to that in [[carbonyl]] compounds, although N<sub>2</sub> is a weaker ''σ''-donor and ''π''-acceptor than CO. Theoretical studies show that ''σ'' donation is a more important factor allowing the formation of the M–N bond than ''π'' back-donation, which mostly only weakens the N–N bond, and end-on (''η''<sup>1</sup>) donation is more readily accomplished than side-on (''η''<sup>2</sup>) donation.<ref name="Greenwood412" />

Today, dinitrogen complexes are known for almost all the [[transition metal]]s, accounting for several hundred compounds. They are normally prepared by three methods:<ref name="Greenwood412" />
# Replacing labile ligands such as [[water|H<sub>2</sub>O]], [[hydride|H<sup>−</sup>]], or [[carbon monoxide|CO]] directly by nitrogen: these are often reversible reactions that proceed at mild conditions.
# Reducing metal complexes in the presence of a suitable co-ligand in excess under nitrogen gas. A common choice includes replacing chloride ligands with [[dimethylphenylphosphine]] (PMe<sub>2</sub>Ph) to make up for the smaller number of nitrogen ligands attached to the original chlorine ligands.
# Converting a ligand with N–N bonds, such as hydrazine or azide, directly into a dinitrogen ligand.
Occasionally the N≡N bond may be formed directly within a metal complex, for example by directly reacting coordinated [[ammonia]] (NH<sub>3</sub>) with [[nitrous acid]] (HNO<sub>2</sub>), but this is not generally applicable. Most dinitrogen complexes have colours within the range white-yellow-orange-red-brown; a few exceptions are known, such as the blue [{Ti(''η''<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>}<sub>2</sub>-(N<sub>2</sub>)].<ref name="Greenwood412" />