Inchemistry, acarbide usually describes acompound composed ofcarbon and a metal. Inmetallurgy,carbiding orcarburizing is the process for producing carbide coatings on a metal piece.[1]
The carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described asinterstitial compounds.[2] These carbides have metallic properties and arerefractory. Some exhibit a range ofstoichiometries, being a non-stoichiometric mixture of various carbides arising due tocrystal defects. Some of them, includingtitanium carbide andtungsten carbide, are important industrially and are used to coat metals in cutting tools.[3]
The long-held 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:[2]
The following table[2][3] shows structures of the metals and their carbides. (N.B. the body centered cubic structure adopted by vanadium, niobium, tantalum, chromium, molybdenum and tungsten is not a close-packed lattice.) The notation "h/2" refers to the M2C 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, although it is technically correct that the carbon atoms fit into the octahedral interstices of a close-packed metal lattice.
| Metal | Structure of pure metal | Metallic radius (pm) | MC metal atom packing | MC structure | M2C metal atom packing | M2C structure | Other carbides |
|---|---|---|---|---|---|---|---|
| titanium | hcp | 147 | ccp | rock salt | |||
| zirconium | hcp | 160 | ccp | rock salt | |||
| hafnium | hcp | 159 | ccp | rock salt | |||
| vanadium | bcc | 134 | ccp | rock salt | hcp | h/2 | V4C3 |
| niobium | bcc | 146 | ccp | rock salt | hcp | h/2 | Nb4C3 |
| tantalum | bcc | 146 | ccp | rock salt | hcp | h/2 | Ta4C3 |
| chromium | bcc | 128 | Cr23C6, Cr3C, Cr7C3, Cr3C2 | ||||
| molybdenum | bcc | 139 | hexagonal | hcp | h/2 | Mo3C2 | |
| tungsten | bcc | 139 | hexagonal | hcp | h/2 |
For a long time thenon-stoichiometric phases were believed to be disordered with a random filling of the interstices, however short and longer range ordering has been detected.[5]
Iron forms a number of carbides,Fe3C,Fe7C3 andFe2C. The best known iscementite, Fe3C, which is present in steels. These carbides are more reactive than the interstitial carbides; for example, the carbides of Cr, Mn, Fe, Co and Ni are all hydrolysed by dilute acids and sometimes by water, to give a mixture of hydrogen and hydrocarbons. These compounds share features with both the inert interstitials and the more reactive salt-like carbides.[2]
Some metals, such aslead andtin, are believed not to form carbides under any circumstances.[6] There exists however a mixed titanium-tin carbide, which is a two-dimensional conductor.[7]
Carbides can be generally classified by the chemical bonds type as follows:
Examples includecalcium carbide (CaC2),silicon carbide (SiC),tungsten carbide (WC; often called, simply,carbide when referring to machine tooling), andcementite (Fe3C),[2] each used in key industrial applications. The naming of ionic carbides is not systematic.
Salt-like carbides are composed of highly electropositive elements such as thealkali metals,alkaline earth metals,lanthanides,actinides, andgroup 3 metals (scandium,yttrium, andlutetium).Aluminium from group 13 formscarbides, butgallium,indium, andthallium do not. These materials feature isolated carbon centers, often described as "C4−", in the methanides or methides; two-atom units, "C2−2", in theacetylides; and three-atom units, "C4−3", in the allylides.[2] The graphite intercalation compound KC8, prepared from vapour of potassium and graphite, and the alkali metal derivatives of C60 are not usually classified as carbides.[8]
Methanides are a subset of carbides distinguished by their tendency to decompose in water producingmethane. Three examples arealuminium carbideAl4C3,magnesium carbideMg2C[9] andberyllium carbideBe2C.
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 oftitanium carbide are hydrolyzed, formingmethane within 5 minutes at ambient conditions, following by saturation of the reaction.[10]
Note that methanide in this context is a trivial historical name. According to the IUPAC systematic naming conventions, a compound such as NaCH3 would be termed a "methanide", although this compound is often called methylsodium.[11] SeeMethyl group#Methyl anion for more information about theCH−3 anion.
Several carbides are assumed to be salts of theacetylide anionC2−2 (also called percarbide, by analogy withperoxide), which has atriple bond between the two carbon atoms. Alkali metals, alkaline earth metals, andlanthanoid metals form acetylides, for example,sodium carbide Na2C2,calcium carbide CaC2, andLaC2.[2] Lanthanides also form carbides (sesquicarbides, see below) with formula M2C3. Metals from group 11 also tend to form acetylides, such ascopper(I) acetylide andsilver acetylide. Carbides of theactinide elements, which have stoichiometry MC2 and M2C3, are also described as salt-like derivatives ofC2−2.
The C–C triple bond length ranges from 119.2 pm in CaC2 (similar to ethyne), to 130.3 pm inLaC2 and 134 pm inUC2. The bonding inLaC2 has been described in terms of LaIII with the extra electron delocalised into the antibonding orbital onC2−2, explaining the metallic conduction.[2]
Thepolyatomic ionC4−3, sometimes calledallylide, is found inLi4C3 andMg2C3. The ion is linear and isisoelectronic with CO2.[2] The C–C distance inMg2C3 is 133.2 pm.[12]Mg2C3 yieldsmethylacetylene,CH3CCH, andpropadiene,CH2CCH2, on hydrolysis, which was the first indication that it containsC4−3.
Carbides of silicon andboron 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.[2]Boron carbide, B4C, on the other hand, has an unusual structure which includes icosahedral boron units linked by carbon atoms. In this respectboron carbide is similar to the boron richborides. Both silicon carbide (also known ascarborundum) and boron carbide are very hard materials andrefractory. Both materials are important industrially. Boron also forms other covalent carbides, such as B25C.
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Metal complexes containing C are known asmetal carbido complexes. Most common are carbon-centered octahedral clusters, such as[Au6C(PPh3)6]2+ (where "Ph" represents aphenyl group) and[Fe6C(CO)6]2−. Similar species are known for themetal carbonyls and the early metal halides. A few terminal carbides have been isolated, such as[CRuCl2(P(C6H11)3)2].
Metallocarbohedrynes (or "met-cars") are stable clusters with the general formulaM8C12 where M is atransition metal (Ti, Zr, V, etc.).
In addition to the carbides, other groups of related carbon compounds exist:[2]
