section_path: Chemistry and compounds/Dinitrogen complexes
section_name: Dinitrogen complexes
paragraph_text: Dinitrogen complexes
  
  The first example of a dinitrogen complex to be discovered was (see figure at right), and soon many other such complexes were discovered. These complexes, in which a nitrogen molecule donates at least one lone pair of electrons to a central metal cation, illustrate how N might bind to the metal(s) in nitrogenase and the catalyst for the Haber process: these processes involving dinitrogen activation are vitally important in biology and in the production of fertilisers.
  
  Dinitrogen is able to coordinate to metals in five different ways. The more well-characterised ways are the end-on M←N≡N (η) and M←N≡N→M (μ, bis- η), 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) or to just one (η). The fifth and unique method involves triple-coordination as a bridging ligand, donating all three electron pairs from the triple bond (μ -N). A few complexes feature multiple N ligands and some feature N bonded in multiple ways. Since N is isoelectronic with carbon monoxide (CO) and acetylene (C H), the bonding in dinitrogen complexes is closely allied to that in carbonyl compounds, although N 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) donation is more readily accomplished than side-on (η) donation.
  
  Today, dinitrogen complexes are known for almost all the transition metals, accounting for several hundred compounds. They are normally prepared by three methods:
  
  Occasionally the N≡N bond may be formed directly within a metal complex, for example by directly reacting coordinated ammonia (NH) with nitrous acid (HNO), 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.