In
chemistry, a
nitride is a compound of
nitrogen where nitrogen has a formal
oxidation state of −3. Nitrides are a large class of compounds with a wide range of properties and applications.
The nitride ion, N3–, is never encountered in solution because it is so basic that it would be protonated. It is isoelectronic with the oxide anion, O2–, and the fluoride anion, F–. Its ionic radius is estimated to be 140 pm. Related to but distinct from nitride is pernitride, N22–.
Uses of nitrides
Like carbides, nitrides are often refractory materials owing to their high lattice energy which reflects the strong attraction of "N3-" for the metal cation. Thus,
titanium nitride and
silicon nitride are used as cutting materials and hard coatings. Hexagonal
boron nitride, which adopts a layered structure, is a useful high temperature lubricant akin to
molybdenum disulfide. Nitride compounds often have large
band gaps, thus nitrides are usually insulators, examples include
boron nitride and silicon nitride. The wide band gap material
gallium nitride is prized for emitting blue light in
LEDs. Like some oxides, nitrides can absorb hydrogen and have been discussed in the context of hydrogen storage, e.g.
lithium nitride.
Examples
Classification of such a varied group of compounds is somewhat arbitrary. Compounds where nitrogen is not assigned 3- oxidation state are not included, e.g.
nitrogen trichloride, nor are ammonia and its many organic derivatives.
Nitrides of the s-block elements
Only one alkali metal nitride is stable, the purple-reddish
lithium nitride (Li
3N), which forms when lithium burns in an atmosphere of N
2.
Sodium nitride and potassium nitride have been generated in the laboratory, however. The nitrides of the alkali earth metals have the formula M
3N and are numerous. Examples include
Mg3N2,
Be3N2,
Ca3N2,
Sr3N2, and Ba
3N
2. The nitrides of electropositive metals (Li, alkali metals Zn) readily hydrolyze upon contact with air:
:2 Mg
4N
3 + 9 H
2O → 8 MgO + 6 NH
3
Nitrides of the p-block elements
Boron nitride exists as several forms (
polymorphs). Nitrides of silicon and phosphorus are also known, but only the former is commercially important. The nitrides of
aluminium,
gallium, and
indium adopt diamond-like
wurtzite structure in which each atom occupies tetrahedral sites. For example in aluminium nitride, each aluminium atom has four neighboring nitrogen atoms at the corners of a tetrahedron and similarly each nitrogen atom has four neighboring aluminium atoms at the corners of a tetrahedron. This structure is like hexagonal diamond (
lonsdaleite) where every carbon atom occupies a tetrahedral site (however wurtzite differs from
sphalerite and
diamond in the relative orientation of tetrahedra). Thallium(I) nitride, Tl
3N is known, but thallium(III) nitride, TlN, is not.
Transition metal nitrides
For the
group 3 metals, scandium nitride (ScN) is known.
group 4,
5, and
6 transition metals, that is the titanium, vanadium and chromium groups all form nitrides. They are
refractory, with high melting point and are chemically stable. Representative is
titanium nitride. Sometimes these materials are called "[[Interstitial defect|interstial] nitrides."
Nitrides of the Group 7 and 8 transition metals decompose readily. For example, iron nitride, Fe2N decomposes at 200 °C. Platinum nitride and osmium nitride may contain N2 units, and as such should not be called nitrides.
The group 11 metals e.g. copper nitride, Cu3N and the group 12 metals e.g. Zn3N2.
Molecular nitrides
Many metals form molecular nitrido complexes, as discussed in the specialized article. The
main group elements also form some molecular nitrides.
Cyanogen ((CN)
2) and
tetrasulfur tetranitride (S
4N
4) are rare examples of a molecular binary (containing one element aside from N) nitrides. They dissolve in nonpolar solvents. Both undergo polymerization. S
4N
4 is also unstable with respect to the elements, but less so that the isostructural Se
4N
4. Heating S
4N
4 gives a polymer, and a variety of molecular sulfur nitride anions and cations are also known.
References
Category:Anions
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