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, N^3–, 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, N₂^2–.

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.^[2] 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.^[3] Sodium nitride and potassium nitride have been generated in the laboratory, however. The nitrides of the alkali earth metals have the formula M₃N₂ and are numerous. Examples include Mg₃N₂, Be₃N₂, Ca₃N₂, Sr₃N₂, and Ba₃N₂. The nitrides of electropositive metals (Li, alkali metals Zn) readily hydrolyze upon contact with air:

2 Mg₄N₃ + 9 H₂O → 8 MgO + 6 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) 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 "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.^[4][5]

The group 11 metals e.g. copper nitride, Cu₃N and the group 12 metals e.g. Zn₃N₂.

Molecular nitrides

Main article: Transition metal nitrido complex

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.

References

1. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0080379419. 
2. ^ The Chemistry of Transition Metal Carbides and Nitrides S. T. Oyama, Ed. Blackie Academic, 1996 ISBN 0751403652. H.O Pierson (1996). Handbook of refractory carbides and nitrides, William Andrew Inc. ISBN 0-8155-1392-5
3. ^ Gregory, Duncan H (2001). "Nitride chemistry of the s-block elements". Coordination Chemistry Reviews 215: 301–345. doi:10.1016/S0010-8545(01)00320-4. 
4. ^ L. Siller, N. Peltekis, S. Krishnamurthy, Y. Chao, S.J. Bull, M.R.C. Hunt (2005). "Gold film with gold nitride-A conductor but harder than gold". Appl. Phys. Lett. 86 (22): 221912. doi:10.1063/1.1941471. 
5. ^ J. A. Montoya, A.D Hernandez, C. Sanloup, E Gregoryanz, S Scandolo (2007). "OsN₂: Crystal structure and electronic properties". Appl. Phys. Lett. 90 (1): 011909. doi:10.1063/1.2430631.