Carbide

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Calcium carbide.

In chemistry, a carbide is a compound of carbon with a more electronegative element. Carbides are important industrially: for example, calcium carbide is a feedstock for the chemical industry and iron carbide, Fe₃C (cementite), is formed in steels to improve their properties.
Many carbides can be generally classified by chemical bonding type as follows:

• 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.

• graphite intercalation compounds
• alkali metal fullerides
• endohedral fullerenes, where the metal atom is encapsulated inside a fullerene molecule
• metallacarbohedrenes(met-cars) which are cluster compounds containing C₂ units.

Examples

Some examples are:-

• Calcium carbide (CaC₂) 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; Fe₃C) 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

• group 1 (the alkali metals )
• group 2 (the alkaline earths )
• group 3 (scandium, yttrium and lanthanum)
• group 11(copper, silver and gold)
• group 12 (zinc ,cadmium and mercury)
• only aluminium from group 13, (gallium, indium and thallium do not appear to form carbides).
• lanthanides when forming MC₂ and M₂C₃ carbides
• actinides when forming MC₂ and M₂C₃ carbides

Most commonly they are salts of C₂²⁻ and are called acetylides, ethynides, acetylenediides or very rarely, percarbides.
Some compounds contain other anionic species:

• C⁴⁻, sometimes called methanides (or methides) because they hydrolyse to give methane gas.
• C₃⁴⁻ 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 C₂²⁻ contains a triple bond between the two carbon atoms. Examples are the carbides of the alkali metals e.g. Na₂C₂, some alkaline earths, e.g. CaC₂ and lanthanoids e.g. LaC₂. The C-C bond distance ranges from 109.2pm in CaC₂ (similar to ethyne), to 130.3 pm in LaC₂ and 134pm in UC₂. The bonding in LaC₂ has been described in terms of La^III with the extra electron delocalised into the antibonding orbital on C₂²⁻, explaining the metallic conduction.

Methanides

The monatomic ion C⁴⁻ is a very strong base, and will combine with four protons to form methane. Methanides commonly react with water to form methane, however reactions with other substances are common.
C⁴⁻ + 4H⁺ → CH₄
Examples of compounds that contain C⁴⁻ are Be₂C and Al₄C₃.

Sesquicarbides

The polyatomic ion C₃⁴⁻ is found in e.g. Li₄C₃, Mg₂C₃. The ion is linear and is isoelectronic with CO₂. The C-C distance in Mg₂C₃ is 133.2 pm. Mg₂C₃ yields methylacetylene, CH₃CCH, on hydrolysis which was the first indication that it may contain C₃⁴⁻.

Covalent carbides

Silicon and boron form covalent carbides. Silicon carbide has two similar crystalline forms, which are both related to the diamond structure. 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, SiC, (carborundum) and boron carbide, B₄C are very hard materials and refractory. Both materials are important industrally. Boron also forms other covalent carbides, e.g. B₂₅C.

Properties

The carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described as interstitial compounds. 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.

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:

• 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, (note that in rock salt, NaCl, it is the chloride anions that are cubic close packed).
• 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 only one of these with carbon achieves 2:1 stoichiometry with the CdI₂ structure.

The following table 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 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.

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	cubic body centered	134	ccp	rock salt	hcp	h/2	V4C3
niobium	cubic body centered	146	ccp	rock salt	hcp	h/2	Nb4C3
tantalum	cubic body centered	146	ccp	rock salt	hcp	h/2	Ta4C3
chromium	cubic body centered	128					Cr23C6, Cr3C, Cr7C3, Cr3C2
molybdenum	cubic body centered	139		hexagonal	hcp	h/2	Mo3C2
tungsten	cubic body centered	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.

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. Multiple stoichiometries are common, for example 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 all are hydrolysed by dilute acids and sometimes by water, to give a mixture of hydrogen and hydrocarbons. These compounds share features with both the inert interstitals and the more reactive salt-like carbides.

References

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