Nitride

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.[1]

The nitride ion, N3−, is never encountered in protic solution because it is so basic that it would be protonated immediately. Its ionic radius is estimated to be 140 pm.

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.[2][3] 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 (Li3N), which forms when lithium burns in an atmosphere of N2.[4] Sodium nitride has been generated, but remains a laboratory curiosity. The nitrides of the alkaline earth metals have the formula M3N2 are however numerous. Examples include Mg3N2, Be3N2, Ca3N2, and Sr3N2. The nitrides of electropositive metals (including Li, Zn, and the alkaline earth metals) readily hydrolyze upon contact with water, including the 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.

S4N4 is a prototypical binary molecular nitride.

Transition metal nitrides

For the group 3 metals, ScN and 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.[5][6]

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.[7]

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 (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.

Related to but distinct from nitride is pernitride, N2−
2.

References

  1. ↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
  2. ↑ Oyama, S. T., ed. (1996). The Chemistry of Transition Metal Carbides and Nitrides. Blackie Academic. ISBN 0-7514-0365-2.
  3. ↑ Pierson, H. O. (1996). Handbook of refractory carbides and nitrides. William Andrew. ISBN 0-8155-1392-5.
  4. ↑ Gregory, Duncan H. (2001). "Nitride chemistry of the s-block elements". Coord. Chem. Rev. 215: 301–345. doi:10.1016/S0010-8545(01)00320-4.
  5. ↑ Siller, L.; Peltekis, N.; Krishnamurthy, S.; Chao, Y.; Bull, S. J.; Hunt, M. R. C. (2005). "Gold film with gold nitride—A conductor but harder than gold". Appl. Phys. Lett. 86 (22): 221912. Bibcode:2005ApPhL..86v1912S. doi:10.1063/1.1941471.
  6. ↑ Montoya, J. A.; Hernández, A. D.; Sanloup, C.; Gregoryanz, E.; Scandolo, S (2007). "OsN2: Crystal structure and electronic properties". Appl. Phys. Lett. 90 (1): 011909. Bibcode:2007ApPhL..90a1909M. doi:10.1063/1.2430631.
  7. ↑ Shanley, Edward S.; Ennis, John L. (1991). "The Chemistry and Free Energy Formation of Silver Nitride". Ind. Eng. Chem. Res. 30 (11): 2503. doi:10.1021/ie00059a023.
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