Carbide

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In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece.[1]
Interstitial / Metallic carbides
Tungsten carbide end mills

The carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described as interstitial compounds.[2] These carbides have metallic properties and are refractory. Some exhibit a range of stoichiometries, being a non-stoichiometric mixture of various carbides arising due to crystal defects. Some of them, including titanium carbide and tungsten 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]

When the metal atoms are cubic close-packed, (ccp), then filling all of the octahedral interstices with carbon achieves 1:1 stoichiometry with the rock salt structure.[4]
When the metal atoms are hexagonal close-packed, (hcp), as the octahedral interstices lie directly opposite each other on either side of the layer of metal atoms, filling only one of these with carbon achieves 2:1 stoichiometry with the CdI2 structure.[4]

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 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.[5]

Iron forms a number of carbides, Fe3C, Fe7C3 and Fe2C. The best known is cementite, 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 as lead and tin, 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]
Chemical classification of carbides

Carbides can be generally classified by the chemical bonds type as follows:

salt-like (ionic),
covalent compounds,
interstitial compounds, and
"intermediate" transition metal carbides.

Examples include calcium carbide (CaC₂), silicon carbide (SiC), tungsten carbide (WC; often called, simply, carbide when referring to machine tooling), and cementite (Fe₃C),^[2] 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 metals, alkaline earth metals, lanthanides, actinides, and group 3 metals (scandium, yttrium, and lutetium). Aluminium from group 13 forms carbides, but gallium, indium, and thallium do not. These materials feature isolated carbon centers, often described as "C⁴⁻", in the methanides or methides; two-atom units, "C2−2", in the acetylides; 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

Methanides are a subset of carbides distinguished by their tendency to decompose in water producing methane. Three examples are aluminium carbide Al₄C₃, magnesium carbide Mg₂C^[9] and beryllium carbide 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.^[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] See Methyl group#Methyl anion for more information about the CH−3 anion.

Acetylides/ethynides
Calcium carbide

Several carbides are assumed to be salts of the acetylide anion C2−2 (also called percarbide, by analogy with peroxide), which has a triple bond between the two carbon atoms. Alkali metals, alkaline earth metals, and lanthanoid metals form acetylides, for example, sodium carbide Na₂C₂, calcium carbide CaC₂, and LaC₂.^[2] Lanthanides also form carbides (sesquicarbides, see below) with formula M₂C₃. Metals from group 11 also tend to form acetylides, such as copper(I) acetylide and silver acetylide. Carbides of the actinide elements, which have stoichiometry MC₂ and M₂C₃, are also described as salt-like derivatives of C2−2.

The C–C triple bond length ranges from 119.2 pm in CaC₂ (similar to ethyne), to 130.3 pm in LaC₂ and 134 pm in UC₂. The bonding in LaC₂ has been described in terms of La^III with the extra electron delocalised into the antibonding orbital on C2−2, explaining the metallic conduction.^[2]

Allylides

The polyatomic ion C4−3, sometimes called allylide, is found in Li₄C₃ and Mg₂C₃. The ion is linear and is isoelectronic with CO₂.^[2] The C–C distance in Mg₂C₃ is 133.2 pm.^[12] Mg₂C₃ yields methylacetylene, CH₃CCH, and propadiene, CH₂CCH₂, on hydrolysis, which was the first indication that it contains C4−3.

Covalent carbides

The 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.^[2] Boron carbide, B₄C, 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 borides. 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 B₂₅C.

.
Molecular carbides
The complex [Au6C(PPh3)6]2+, containing a carbon-gold core

Metal complexes containing C are known as metal carbido complexes. Most common are carbon-centered octahedral clusters, such as [Au₆C(PPh₃)₆]²⁺ (where "Ph" represents a phenyl group) and [Fe₆C(CO)₆]²⁻. Similar species are known for the metal 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 formula M₈C₁₂ where M is a transition metal (Ti, Zr, V, etc.).

.
Related materials

In addition to the carbides, other groups of related carbon compounds exist:[2]

• graphite intercalation compounds
• alkali metal fullerides
• endohedral fullerenes, where the metal atom is encapsulated within a fullerene molecule
• metallacarbohedrenes (met-cars) which are cluster compounds containing C₂ units.
• tunable nanoporous carbon, where gas chlorination of metallic carbides removes metal molecules to form a highly porous, near-pure carbon material capable of high-density energy storage.
• transition metal carbene complexes.
• two-dimensional transition metal carbides: MXenes

See also

Kappa-carbides

References

Kunst, Helmut; Haase, Brigitte; Malloy, James C.; Wittel, Klaus; Nestler, Montia C.; Nicoll, Andrew R.; Erning, Ulrich; Rauscher, Gerhard (2006). "Metals, Surface Treatment". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a16_403.pub2.
Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 318–22. ISBN 978-0-08-022057-4.
Peter Ettmayer; Walter Lengauer (1994). "Carbides: transition metal solid state chemistry". In R. Bruce King (ed.). Encyclopedia of Inorganic Chemistry. John Wiley & Sons. ISBN 978-0-471-93620-6.
Zhu, Qinqing; Xiao, Guorui; Cui, Yanwei; Yang, Wuzhang; Wu, Siqi; Cao, Guang-Han; Ren, Zhi (2021-10-15). "Anisotropic lattice expansion and enhancement of superconductivity induced by interstitial carbon doping in Rhenium". Journal of Alloys and Compounds. 878: 160290. doi:10.1016/j.jallcom.2021.160290. ISSN 0925-8388.
C.H. de Novion; J.P. Landesman (1985). "Order and disorder in transition metal carbides and nitrides: experimental and theoretical aspects". Pure Appl. Chem. 57 (10): 1391. doi:10.1351/pac198557101391. S2CID 59467042.
John Percy (1870). The Metallurgy of Lead, including Desiverization and Cupellation. London: J. Murray. p. 67. Retrieved 2013-04-06.
Y. C. Zhou; H. Y. Dong; B. H. Yu (2000). "Development of two-dimensional titanium tin carbide (Ti2SnC) plates based on the electronic structure investigation". Materials Research Innovations. 4 (1): 36–41. Bibcode:2000MatRI...4...36Z. doi:10.1007/s100190000065. S2CID 135756713.
Shriver and Atkins — Inorganic Chemistry
O.O. Kurakevych; T.A. Strobel; D.Y. Kim; G.D. Cody (2013). "Synthesis of Mg2C: A Magnesium Methanide". Angewandte Chemie International Edition. 52 (34): 8930–8933. doi:10.1002/anie.201303463. PMID 23824698.
A. I. Avgustinik; G. V. Drozdetskaya; S. S. Ordan'yan (1967). "Reaction of titanium carbide with water". Powder Metallurgy and Metal Ceramics. 6 (6): 470–473. doi:10.1007/BF00780135. S2CID 134209836.
Weiss, Erwin; Corbelin, Siegfried; Cockcroft, Jeremy Karl; Fitch, Andrew Nicholas (1990). "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". Chemische Berichte. 123 (8): 1629–1634. doi:10.1002/cber.19901230807. ISSN 0009-2940.

Fjellvag H.; Pavel K. (1992). "Crystal Structure of Magnesium Sesquicarbide". Inorg. Chem. 31 (15): 3260. doi:10.1021/ic00041a018.

Salts and covalent derivatives of the carbide ion

CH4 +H He Li2C2 Be2C B4C BnCm +B C C2 C4− CN− (CN)2 +N CO CO2 C3O2 CF CF4 Ne Na2C2 Mg2C Al4C3 SiC +Si +P CS2 +S CCl4 +Cl Ar K2C2 CaC CaC2 ScC Sc3C4 Sc4C3 Sc15C19 TiC VC Cr3C2 MnC2 Fe2C Fe3C Fe5C2 CoC Ni2C CuC CuC2 Zn2C Ga +Ge +As CSe2 CBr4 +Br Kr Rb2C2 SrC2 YC ZrC NbC MoC Mo2C Tc Ru2C Rh2C PdC2 Ag2C2 CdC InC Sn Sb Te CI4 +I Xe Cs2C2 BaC2 * LuC2 HfC TaC5 WC Re2C Os2C Ir2C PtC Au2C2 Hg2C2 TlC ?PbC Bi Po At Rn Fr Ra ** Lr Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og   * LaC2 CeC2 PrC2 NdC2 PmC2 SmC2 EuC2 GdC2 TbC2 DyC2 HoC2 ErC2 TmC2 YbC2 ** Ac ThC PaC UC NpC, Np2C3 PuC, Pu2C3 Am Cm Bk Cf Es Fm Md No

Inorganic compounds of carbon and related ions

Inorganic compounds of carbon and related ions
Compounds	CF CO CO2 CO3 CO4 CO5 CO6 COS CS C2S2 CS2 CSe2 C3O2 C3S2 SiC
Carbon ions	Carbides [:C≡C:]2−, [::C::]4−, [:C=C=C:]4− Cyanide [:C≡N:]− Cyanate [:O−C≡N:]− Thiocyanate [:S−C≡N:]− Fulminate [:C≡N−O:]− Thiofulminate [:C≡N−S:]−
Nanostructures	Graphite intercalation compounds Fullerides
Oxides and related	Oxides Nitrides Metal carbonyls Carbonic acid Bicarbonates Carbonates

Monatomic anion compounds
Group 1	H−
Group 13	B Al Ga In Tl
Group 14	C Si Ge Sn Pb
Group 15 (Pnictides)	N3− P3− As3− Sb3− Bi3−
Group 16 (Chalcogenides)	O2− S2− Se2− Te2− Po2−
Group 17 (Halides)	F− Cl− Br− I− At−

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