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==Overview==
[[Image:Carbid.jpg|200px|thumb|Calcium carbide.]]
In [[chemistry]], a '''carbide''' is a compound of [[carbon]] with a more  [[electronegativity|electronegative]] element. Carbides are important industrially: for example, [[calcium carbide]] is a feedstock for the chemical industry and iron carbide, Fe<sub>3</sub>C ([[cementite]]), is formed in steels to improve their properties.
<br />
Many carbides can be generally classified by chemical bonding type as follows<ref name= "Greenwood"> {{Greenwood&Earnshaw}}</ref>:

* salt-like ionic compounds
* covalent compounds 
* interstitial compounds
* "intermediate" transition metal carbides (a group of carbides that in bonding terms sit between the salt-like and interstitial carbides).
  
In addition to the carbides there are other groups of binary carbon compounds, i.e.<ref name= "Greenwood"/> 
*[[graphite intercalation compound]]s
*alkali metal fullerides
*[[endohedral fullerenes]], where the metal atom is encapsulated inside a fullerene molecule
*metallacarbohedrenes(met-cars) which are cluster compounds containing C<sub>2</sub> units.

==Examples==
Some examples are<ref name= "Greenwood"/>:-
* [[Calcium carbide]] (CaC<sub>2</sub>) important industrially and an ionic salt 
* [[Silicon carbide]] (SiC), carborundum, a covalent compound
* [[Tungsten carbide]] (often called simply ''carbide'') widely used for cutting tools and an interstitial compound
* [[Cementite]] (iron carbide; Fe<sub>3</sub>C) an important constituent of steel
* [[Boron carbide]]
* [[Tantalum carbide]]
* [[Titanium carbide]]

See [[:Category:Carbides]] for a bigger list.

==Types of carbides==

===Ionic salts===
Salt like carbides are formed by the metals of<ref name= "Greenwood"/> 
*group 1 (the [[alkali metal]]s )
*group 2 (the [[alkaline earth]]s )
*group 3 ([[scandium]], [[yttrium]] and [[lanthanum]])
*group 11([[copper(I) acetylide|copper]], [[silver acetylide|silver]] and [[gold]])
*group 12 ([[zinc]] ,[[cadmium]] and [[mercury (element)|mercury]])
*only [[aluminium carbide|aluminium]] from group 13, ([[gallium]], [[indium]] and [[thallium]] do not appear to form carbides).
*[[Lanthanoid|lanthanides]] when forming MC<sub>2</sub> and M<sub>2</sub>C<sub>3</sub> carbides
*[[actinides]] when forming MC<sub>2</sub> and M<sub>2</sub>C<sub>3</sub> carbides
Most commonly they are salts of C<sub>2</sub><sup>2<nowiki>&minus;</nowiki></sup> and are called acetylides, ethynides, acetylenediides or very rarely, percarbides. <br />
Some compounds contain other anionic species:<ref name= "Greenwood"/>
*C<sup>4<nowiki>&minus;</nowiki></sup>, sometimes called methanides (or methides) because they hydrolyse to give [[methane]] gas.
*C<sub>3</sub><sup>4<nowiki>&minus;</nowiki></sup> ion, sometimes called sesquicarbides, they hydrolyse to give [[methylacetylene]].
The naming of ionic carbides is not consistent and can be quite confusing.
====Acetylides====
The [[polyatomic ion]] C<sub>2</sub><sup>2<nowiki>&minus;</nowiki></sup> contains a [[covalent bond|triple bond]] between the two carbon atoms. Examples are the carbides of the alkali metals e.g. Na<sub>2</sub>C<sub>2</sub>, some alkaline earths, e.g. [[calcium carbide|CaC<sub>2</sub>]] and lanthanoids e.g. [[lanthanum carbide|LaC<sub>2</sub>]].<ref name= "Greenwood"/> The C-C bond distance ranges from 109.2pm in [[calcium carbide|CaC<sub>2</sub>]] (similar to ethyne), to 130.3 pm in [[lanthanum carbide|LaC<sub>2</sub>]] and 134pm in [[uranium carbide|UC<sub>2</sub>]].<ref name= "Greenwood"/> The bonding in [[lanthanum carbide|LaC<sub>2</sub>]] has been described in terms of La<sup>III</sup> with the extra electron delocalised into the antibonding orbital on C<sub>2</sub><sup>2<nowiki>&minus</nowiki>;</sup>, explaining the metallic conduction.<ref name= "Greenwood"/>

====Methanides====
The [[monatomic ion]] C<sup>4<nowiki>&minus;</nowiki></sup> is a very strong base, and will combine with four [[proton]]s to form [[methane]]. Methanides commonly react with water to form [[methane]], however reactions with other substances are common.
<br />
C<sup>4<nowiki>&minus;</nowiki></sup> + 4H<sup>+</sup> → CH<sub>4</sub></br>
Examples of compounds that contain C<sup>4<nowiki>&minus;</nowiki></sup> are Be<sub>2</sub>C and [[aluminium carbide|Al<sub>4</sub>C<sub>3</sub>]].<ref name= "Greenwood"/>

====Sesquicarbides====
The [[polyatomic ion]] C<sub>3</sub><sup>4<nowiki>&minus;</nowiki></sup> is found in e.g. [[lithium carbide|Li<sub>4</sub>C<sub>3</sub>]], Mg<sub>2</sub>C<sub>3</sub>.<ref name= "Greenwood"/> The ion is linear and is isoelectronic with CO<sub>2</sub>.<ref name= "Greenwood"/> The C-C distance in Mg<sub>2</sub>C<sub>3</sub> is 133.2 pm.<ref>''Crystal Structure of Magnesium Sesquicarbide'' Fjellvag H. and Pavel K. Inorg. Chem. 1992, 31, 3260</ref> Mg<sub>2</sub>C<sub>3</sub> yields [[methylacetylene]], CH<sub>3</sub>CCH, on hydrolysis which was the first indication that it may contain C<sub>3</sub><sup>4<nowiki>&minus;</nowiki></sup>.

=== Covalent carbides===
Silicon and boron form covalent carbides.<ref name= "Greenwood"/> [[Silicon carbide]] has two similar crystalline forms, which are both related to the diamond structure.<ref name= "Greenwood"/> [[Boron carbide]], B<sub>4</sub>C, on the other hand has an unusual structure which includes icosahedral boron units linked by carbon atoms. In this respect [[boron carbide]] is similar to the boron rich [[boride]]s.  Both [[silicon carbide]], SiC, (carborundum) and [[boron carbide]], B<sub>4</sub>C are very hard materials and [[refractory]]. Both materials are important industrally. Boron also forms other covalent carbides, e.g. B<sub>25</sub>C.

===Interstitial carbides===
===Properties===
The carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described as [[interstitial compound]]s.<ref name= "Greenwood"/> These carbides are chemically quite inert, have metallic properties and are [[refractory]]. Some exhibit a range of stoichiometries, e.g. [[titanium carbide]], TiC. [[Titanium carbide]] and [[tungsten carbide]] are important industrially and are used to coat metals in cutting tools.<ref name = "Ettmayer">''Carbides: transition metal solid state chemistry'' Peter Ettmayer & Walter Lengauer, Encyclopedia of Inorganic Chemistry Editor in chief  R. Bruce King Pub 1994 John Wiley & Sons ISBN 0-471-93620-0</ref>

===Structure===
The longheld view is that the carbon atoms fit into octahedral interstices in a close packed metal lattice when the metal atom radius is greater than approximately 135 pm:<ref name= "Greenwood"/> 
* When the metal atoms are [[close-packing|cubic close packed]], (ccp), then filling all of the octahedral interstices with carbon achieves 1:1 stoichiometry with the rock salt structure, (note that in rock salt, NaCl, it is the chloride anions that are [[close-packing|cubic close packed]]). 
* When the metal atoms are [[close-packing|hexagonal close packed]], (hcp), as the octahedral interstices lie directly opposite each other on either side of the layer of metal atoms, filling only only one of these with carbon achieves 2:1 stoichiometry with the CdI<sub>2</sub> structure.

The following table <ref name= "Greenwood"/><ref name = "Ettmayer"/>shows actual structures of the metals and their carbides. (N.B. the body centred cubic structure adopted by vanadium, niobium, tantalum, chromium, molybdenum and tungsten is not a close packed lattice.) The notation "h/2" refers to the M<sub>2</sub>C type structure described above, which is only an approximate description of the actual structures. The simple view that the lattice of the pure metal "absorbs" carbon atoms can be seen to be untrue as the packing of the metal atom lattice in the carbides is different from the packing in the pure metal.

{| class="wikitable"
|-
! Metal
! Structure of pure metal
! Metallic <br />radius (pm)
! "align = center"|MC <br />- metal atom packing
! MC structure
! M<sub>2</sub>C <br />- metal atom packing
! M<sub>2</sub>C structure
! Other carbides
|-
| [[titanium]]
| align = "center" |hcp
| align = "center" |147
| align = "center"|ccp
| align = "center" |rock salt
| 
|
|
|-
| [[zirconium]]
| align = "center" |hcp
| align = "center" |160
| align = "center" |ccp
| align = "center" |rock salt 
|
|
|
|-
| [[hafnium]]
| align = "center" |hcp
| align = "center" |159
| align = "center" |ccp
| align = "center" |rock salt
|
|
|
|-
| [[vanadium]]
| align = "center" |cubic body centered
| align = "center" |134
| align = "center" |ccp
| align = "center" |rock salt
| align = "center" |hcp
| align = "center" |h/2
| align = "center" |V<sub>4</sub>C<sub>3</sub>
|-
| [[niobium]]
| align = "center" |cubic body centered
| align = "center" | 146
| align = "center" |ccp
| align = "center" |rock salt
| align = "center" | hcp
| align = "center" | h/2
| align = "center" |Nb<sub>4</sub>C<sub>3</sub>
|-
| [[tantalum]]
| align = "center" |cubic body centered
| align = "center" | 146
| align = "center" |ccp
| align = "center" |rock salt
| align = "center" |hcp
| align = "center" |h/2
| align = "center" |Ta<sub>4</sub>C<sub>3</sub>
|-
| [[chromium]]
| align = "center" |cubic body centered
| align = "center" |128
|
|
| 
| 
|align = "center" |Cr<sub>23</sub>C<sub>6</sub>, Cr<sub>3</sub>C,<br /> Cr<sub>7</sub>C<sub>3</sub>, Cr<sub>3</sub>C<sub>2</sub>
|-
| [[molybdenum]]
| align = "center" |cubic body centered
| align = "center" |139
|
| align = "center" |hexagonal
| align = "center" |hcp
| align = "center" |h/2
| align = "center" |Mo<sub>3</sub>C<sub>2</sub>
|-
| [[tungsten]]
| align = "center" |cubic body centered
| align = "center" |139
|
| align = "center" |hexagonal
| align = "center" |hcp
| align = "center" |h/2
| 
|}

For a long time the non stoichiometric phases were believed to be disordered with a random filling of the interstices, however short and longer range ordering has been detected<ref> ''Order and disorder in transition metal carbides and nitrides: experimental and theoretical aspects'' C.H. de Novion and J.P. Landesman Pure & Appl. Chem., 57, 10,(1985)1391</ref>.

===Intermediate transition metal carbides===
In these the transition metal ion is smaller than the critical 135 pm and the structures are not interstitial but are more complex. <ref name= "Greenwood"/> Multiple stoichiometries are common, for example iron forms a number of carbides, Fe<sub>3</sub>C,  Fe<sub>7</sub>C<sub>3</sub> and Fe<sub>2</sub>C.<ref name= "Greenwood"/> The best known is [[cementite]], Fe<sub>3</sub>C, which is present in steels.<ref name= "Greenwood"/>
These carbides are more reactive than the interstitial carbides, for example the carbides of Cr, Mn, Fe, Co and Ni all are  hydrolysed by dilute acids and sometimes by water, to give a mixture of hydrogen and hydrocarbons.<ref name= "Greenwood"/> These compounds share features with both the inert interstitals and the more reactive salt-like carbides.<ref name= "Greenwood"/>

==References==
{{reflist|2}}

{{Inorganic compounds of carbon}}


[[Category:Anions]]
[[Category:Salts]]

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