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⁻ and azides, that contain N₃⁻.
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.

• 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

Classification of such a varied group of compounds is necessarily arbitrary. The following is based around their structure:

• 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

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Nitride ion

The nitride ion is N³⁻ (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⁻, and the fluoride anion, F⁻ and has an ionic radius estimated to be 140 pm. The nitride ion is a strong π-donor ligand, stronger than O²⁻. 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 ^[1] and K₃N ^[2] have been synthesised by simultaneously depositing metal atoms and nitrogen atoms onto a liquid nitrogen cooled sapphire substrate. Both are unstable compounds.

• 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

Lithium nitride and the alkaline earth nitrides deprotonate hydrogen gas, and are rapidly hydrolysed by water to form ammonia.

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 200^oC. 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. ^{[3] [4]}

General references

• WebElements
• Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. 
• H.O Pierson (1996). Handbook of refractory carbides and nitrides, William Andrew Inc. ISBN 0-8155-1392-5