Nitride

In chemistry a nitride is a compound of nitrogen with a less electronegative element where nitrogen has an oxidation state of -3. Note that there are exceptions to this naming convention, the nitrides of hydrogen, NH3 and carbon, (CN)2, are called ammonia and cyanogen respectively and that the nitrides of bromine, iodine are called nitrogen tribromide and nitrogen triiodide. Note that nitrogen also forms pernitrides, that contain N22 &minus; and azides, that contain N3&minus;. Nitrogen has one of the highest electronegativities, only oxygen, fluorine and chlorine are higher. This means that the nitrides are a very large group of compounds. They have wide range of properties and applications. Classification of such a varied group of compounds is necessarily arbitrary. The following is based around their structure:
 * refractory materials e.g.
 * lubricant e.g. hexagonal boron nitride, BN
 * cutting materials e.g. silicon nitride, Si3N4
 * insulators e.g. boron nitride, BN, silicon nitride, Si3N4
 * semiconductors e.g. gallium nitride, GaN
 * metal coatings e.g. titanium nitride, TiN
 * hydrogen storage e.g Lithium nitride, Li3N
 * salt like, e.g. lithium nitride, Li3N, beryllium nitride, Be3N2
 * covalent
 * 3 dimensional structures e.g. phosphorus nitride, P3N5; boron nitride, BN
 * diamond like e.g. gallium nitride, GaN
 * molecular ("volatile") e.g. tetrasulfur tetranitride, S4N4
 * interstitial e.g. titanium nitride, TiN
 * intermediate e.g. iron nitride, Fe2N

Nitride ion
The nitride ion is N3&minus; (a nitrogen atom plus three electrons). The extra electrons give the nitrogen atom a closed inert gas shell. The nitride ion is isoelectronic with the oxide anion, O2 &minus;, and the fluoride anion, F&minus; and has an ionic radius estimated to be 140 pm. The nitride ion is a strong π-donor ligand, stronger than O2&minus;. It forms nitrido complexes which have a short metal nitrogen bond length indicating multiple bonding.

Salt like nitrides
The salt like nitrides are formed by:
 * the alkali metals, Li3N, Na3N and K3N. Li3N is readily formed and has a unique structure. Na3N and K3N have been synthesised by simultaneously depositing metal atoms and nitrogen atoms onto a liquid nitrogen cooled sapphire substrate. Both are unstable compounds.

Lithium nitride and the alkaline earth nitrides deprotonate hydrogen gas, and are rapidly hydrolysed by water to form ammonia.
 * the alkaline earth metals Mg3N2, Be3N2 and Ca3N2
 * the group 3 metals e.g. scandium nitride, ScN
 * the group 11 metals e.g. copper nitride, Cu3N
 * the group 12 metals e.g. Zn3N2

Covalent nitrides

 * 3 dimensional structures
 * These include, boron nitride silicon and phosphorus.


 * Diamond like nitrides
 * The diamond like nitrides of aluminium, gallium and indium all have the wurtzite structure in which each atom occupies tetrahedral sites. For example in aluminium nitride, each aluminium atom has four neighbouring nitrogen atoms at the corners of a tetrahedron and similarly each nitrogen atom has four neighbouring aluminium atoms at the corners of a tetrahedron. This structure is like hexagonal diamond (Lonsdaleite) where every carbon atom occupies a tetrahedral site (however wurzite differs from sphalerite and diamond in the relative orientation of tetrahedra) Note that thallium(III) nitride, TlN is not known, whereas thallium(I) nitride, Tl3N is.


 * Molecular
 * These include cyanogen, (CN)2 and S2N2 and tetrasulfur tetranitride, S4N4. (Note that sulfur forms another nitride which is polymeric, (SN)x, this is a metallic conductor and has been called a one-dimensional metal.)

Interstitial nitrides
The interstitial nitrides are formed by transition metals where there is a sufficient difference in size between the metal atom and the nitrogen to allow the host metal lattice to accommodate the nitrogen atom. This condition is true for the group 4, 5 and 6 transition metals i.e. the Titanium, Vanadium and Chromium groups. The group 4 and 5 nitrides are refractory i.e. high melting and chemically stable.

Intermediate nitrides
Group 7 and 8 transition metals form nitrides that decompose readily e.g iron nitride, Fe2N melts with decomposition at 200oC. The precious metals are currently being investigated by a number of researchers and thin films of platinum, gold and osmium nitrides have been produced. However there is some discussion as to their structures and their properties. Platinum nitride and osmium nitride for example are now believed to contain N2 units and as such should not be called nitrides.

General references

 * WebElements
 * H.O Pierson (1996). Handbook of refractory carbides and nitrides, William Andrew Inc. ISBN 0-8155-1392-5
 * H.O Pierson (1996). Handbook of refractory carbides and nitrides, William Andrew Inc. ISBN 0-8155-1392-5