Editing Carbide (section)

==Chemical classification of carbides== 
Carbides can be generally classified by the chemical bonds type as follows: 
# salt-like (ionic),
# [[covalent compound]]s,
# [[interstitial compound]]s, and
# "intermediate" [[transition metal]] carbides.
Examples include [[calcium carbide]] (CaC<sub>2</sub>), [[silicon carbide]] (SiC), [[tungsten carbide]] (WC; often called, simply, ''carbide'' when referring to machine tooling), and [[cementite]] (Fe<sub>3</sub>C),<ref name="Greenwood">{{Greenwood&Earnshaw1st|pages=318–22}}</ref> each used in key industrial applications. The naming of ionic carbides is not systematic.

===Salt-like / saline / ionic carbides===
Salt-like carbides are composed of highly electropositive elements such as the [[alkali metal]]s, [[alkaline earth metal]]s, [[lanthanide]]s, [[actinide]]s, and [[group 3 element|group 3 metals]] ([[scandium]], [[yttrium]], and [[lutetium]]). [[Aluminium]] from group 13 forms [[Aluminium carbide|carbides]], but [[gallium]], [[indium]], and [[thallium]] do not. These materials feature isolated carbon centers, often described as "C<sup>4−</sup>", in the methanides or methides; two-atom units, "{{chem2|C2(2-)}}", in the [[acetylide]]s; and three-atom units, "{{chem2|C3(4-)}}", in the allylides.<ref name="Greenwood" /> The [[graphite intercalation compound | graphite intercalation compound KC<sub>8</sub>]], prepared from vapour of potassium and graphite, and the alkali metal derivatives of C<sub>60</sub> are not usually classified as carbides.<ref>Shriver and Atkins&nbsp;— Inorganic Chemistry</ref>

====Methanides====
Methanides are a subset of carbides distinguished by their tendency to decompose in water producing [[methane]]. Three examples are [[aluminium carbide]] {{chem2|Al4C3}}, [[magnesium carbide]] {{chem2|Mg2C}}<ref>{{cite journal|title=Synthesis of Mg2C: A Magnesium Methanide|author1=O.O. Kurakevych |author2=T.A. Strobel |author3=D.Y. Kim |author4=G.D. Cody |volume =52|issue=34|year=2013|pages=8930–8933|journal=Angewandte Chemie International Edition|doi=10.1002/anie.201303463|pmid = 23824698}}</ref> and [[beryllium carbide]] {{chem2|Be2C}}.

Transition metal carbides are not saline: their reaction with water is very slow and is usually neglected. For example, depending on surface porosity, 5–30 atomic layers of [[titanium carbide]] are hydrolyzed, forming [[methane]] within 5 minutes at ambient conditions, following by saturation of the reaction.<ref>{{cite journal|url=https://link.springer.com/article/10.1007%2FBF00780135|title=Reaction of titanium carbide with water|author1=A. I. Avgustinik |author2=G. V. Drozdetskaya |author3=S. S. Ordan'yan |volume =6|issue=6|year=1967|pages=470–473|journal=Powder Metallurgy and Metal Ceramics|doi=10.1007/BF00780135|s2cid=134209836}}</ref>

Note that methanide in this context is a trivial historical name. According to the IUPAC systematic naming conventions, a compound such as NaCH<sub>3</sub> would be termed a "methanide", although this compound is often called methylsodium.<ref name="WeissCorbelin1990">{{cite journal|last1=Weiss|first1=Erwin|last2=Corbelin|first2=Siegfried|last3=Cockcroft|first3=Jeremy Karl|last4=Fitch|first4=Andrew Nicholas|title=Über Metallalkyl- und -aryl-Verbindungen, 44 Darstellung und Struktur von Methylnatrium. Strukturbestimmung an NaCD3-Pulvern bei 1.5 und 300 K durch Neutronen- und Synchrotronstrahlenbeugung|journal=Chemische Berichte|volume=123|issue=8|year=1990|pages=1629–1634|issn=0009-2940|doi=10.1002/cber.19901230807}}</ref> See [[Methyl group#Methyl anion]] for more information about the {{chem2|CH3-}} anion.

====Acetylides/ethynides====
[[Image:Carbid.jpg|thumb|[[Calcium carbide]]]]
Several carbides are assumed to be salts of the [[acetylide|acetylide anion]] {{chem2|C2(2–)}} (also called percarbide, by analogy with [[peroxide]]), which has a [[covalent bond|triple bond]] between the two carbon atoms. Alkali metals, alkaline earth metals, and [[lanthanoid|lanthanoid metals]] form acetylides, for example, [[sodium carbide]] Na<sub>2</sub>C<sub>2</sub>, [[calcium carbide]] CaC<sub>2</sub>, and [[lanthanum carbide|LaC<sub>2</sub>]].<ref name="Greenwood" /> Lanthanides also form carbides (sesquicarbides, see below) with formula M<sub>2</sub>C<sub>3</sub>. Metals from group 11 also tend to form acetylides, such as [[copper(I) acetylide]] and [[silver acetylide]]. Carbides of the [[actinides|actinide elements]], which have stoichiometry MC<sub>2</sub> and M<sub>2</sub>C<sub>3</sub>, are also described as salt-like derivatives of {{chem2|C2(2-)}}.

The C–C triple bond length ranges from 119.2&nbsp;pm in CaC<sub>2</sub> (similar to ethyne), to 130.3&nbsp;pm in [[lanthanum carbide|LaC<sub>2</sub>]] and 134&nbsp;pm in [[uranium carbide|UC<sub>2</sub>]]. 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 {{chem2|C2(2-)}}, explaining the metallic conduction.<ref name="Greenwood" />

====Allylides====
The [[polyatomic ion]] {{chem2|C3(4−)}}, sometimes called '''allylide''', is found in {{chem2|Li4C3}} and {{chem2|Mg2C3}}. The ion is linear and is [[isoelectronic]] with {{CO2}}.<ref name="Greenwood" /> The C–C distance in {{chem2|Mg2C3}} is 133.2&nbsp;pm.<ref>{{cite journal|title=Crystal Structure of Magnesium Sesquicarbide|doi=10.1021/ic00041a018|author1=Fjellvag H. |author2=Pavel K. |journal=Inorg. Chem. |year=1992|volume=31|page=3260|issue=15}}</ref> {{chem2|Mg2C3}} yields [[methylacetylene]], {{chem2|CH3CCH}}, and [[propadiene]], {{chem2|CH2CCH2}}, on hydrolysis, which was the first indication that it contains {{chem2|C3(4−)}}.

===Covalent carbides===
Carbides of silicon and [[boron]] are described as "covalent carbides", although virtually all compounds of carbon exhibit some covalent character. [[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 (also known as ''carborundum'') and boron carbide are very hard materials and [[refractory]]. Both materials are important industrially. Boron also forms other covalent carbides, such as B<sub>25</sub>C.

===Molecular carbides===
[[Image:Au6C(PPh3)6.png|thumb|right|The complex {{chem2|[Au6C(PPh3)6](2+)}}, containing a carbon-gold core]]
Metal complexes containing C are known as [[metal carbido complex]]es. Most common are carbon-centered octahedral clusters, such as {{chem2|[Au6C(P[[Ph]]3)6](2+)}} (where "Ph" represents a [[phenyl group]]) and {{chem2|[Fe6C(CO)6](2−)}}. Similar species are known for the [[metal carbonyl]]s and the early metal halides. A few terminal carbides have been isolated, such as {{chem2|[CRuCl2(P(C6H11)3)2]}}.

[[Metallocarbohedryne]]s (or "met-cars") are stable clusters with the general formula {{chem2|M8C12}} where M is a [[transition metal]] (Ti, Zr, V, etc.).