Nitric Oxide Evolution 2 Hemoglobin

The chemical element nitrogen is one of the most abundant elements in the universe and can form many compounds. It can take several oxidation states; but the most common oxidation states are −3 and +3. Nitrogen can form nitride and nitrate ions. It also forms a part of nitric acid and nitrate salts. Nitrogen compounds also have an important role in organic chemistry, as nitrogen is part of proteins, amino acids and adenosine triphosphate.

Dinitrogen complexes

The first example of a dinitrogen complex to be discovered was u(NH₃)₅(N₂)sup>2+ (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 N₂ 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''¹) and M←N≡N→M ('' μ'', bis-''η''¹), 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''²) or to just one (''η''²). The fifth and unique method involves triple-coordination as a bridging ligand, donating all three electron pairs from the triple bond (''μ''₃-N₂). A few complexes feature multiple N₂ ligands and some feature N₂ bonded in multiple ways. Since N₂ is isoelectronic with carbon monoxide (CO) and acetylene (C₂H₂), the bonding in dinitrogen complexes is closely allied to that in carbonyl compounds, although N₂ 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''¹) donation is more readily accomplished than side-on (''η''²) donation.

Today, dinitrogen complexes are known for almost all the transition metals, accounting for several hundred compounds. They are normally prepared by three methods:
# Replacing labile ligands such as H₂O, H⁻, or CO directly by nitrogen: these are often reversible reactions that proceed at mild conditions.
# Reducing metal complexes in the presence of a suitable coligand in excess under nitrogen gas. A common choice include replacing chloride ligands by dimethylphenylphosphine (PMe₂Ph) to make up for the smaller number of nitrogen ligands attached than 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 (NH₃) with nitrous acid (HNO₂), 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 sub>2-(N₂)

Nitrides, azides, and nitrido complexes

Nitrogen bonds to almost all the elements in the periodic table except the first three noble gases, helium, neon, and argon, and some of the very short-lived elements after bismuth, creating an immense variety of binary compounds with varying properties and applications. Many binary compounds are known: with the exception of the nitrogen hydrides, oxides, and fluorides, these are typically called nitrides. Many stoichiometric phases are usually present for most elements (e.g. MnN, Mn₆N₅, Mn₃N₂, Mn₂N, Mn₄N, and Mn_''x''N for 9.2 < ''x'' < 25.3). They may be classified as "salt-like" (mostly ionic), covalent, "diamond-like", and metallic (or interstitial