The nitride ion, N3−, is never encountered in solution because it is so basic that it would be protonated. Its ionic radius is estimated to be 140 pm. Related to but distinct from nitride is pernitride, N2−
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 or wide bandgap semiconductors, 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.
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 (Li3N), which forms when lithium burns in an atmosphere of N2. Sodium nitride and potassium nitride have been generated in the laboratory, however. The nitrides of the alkaline earth metals have the formula M3N2 and are numerous. Examples include Mg3N2, Be3N2, Ca3N2, Sr3N2, and Ba3N2. The nitrides of electropositive metals (including Li, Zn, and the alkaline earth metals) readily hydrolyze upon contact with water (e.g. moisture in the air):
- Mg3N2 + 6 H2O → 3 Mg(OH)2 + 2 NH3
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, Tl3N is known, but thallium(III) nitride, TlN, is not.
Transition metal nitrides
For the group 3 metals, scandium nitride (ScN) and yttrium nitride (YN) are both 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 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.
Nitrides of heavier members from group 11 and 12 are less stable than copper nitride, Cu3N and Zn3N2: dry silver nitride (Ag3N) is a contact explosive which may detonate from the slightest touch, even a falling water droplet.
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 (S4N4) are rare examples of a molecular binary (containing one element aside from N) nitrides. They dissolve in nonpolar solvents. Both undergo polymerization. S4N4 is also unstable with respect to the elements, but less so that the isostructural Se4N4. Heating S4N4 gives a polymer, and a variety of molecular sulfur nitride anions and cations are also known.
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