Carbide

Lattice structure of titanium carbide.

In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by chemical bonding type as follows: (i) salt-like, (ii) covalent compounds, (iii) interstitial compounds, and (iv) "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),^[1] each used in key industrial applications. The naming of ionic carbides is not systematic.

Salt-like (saline) carbides

Salt-like carbides are composed of highly electropositive elements such as the alkali metals, alkaline earth metals, and group 3 metals including scandium, yttrium and lanthanum. 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, "C₂²⁻", in the acetylides; and three-atom units, "C₃⁴⁻", in the sesquicarbides.^[1] 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.^[2]

Methanides

Tungsten carbide end mills.

Carbides of this class decompose in water producing methane. Three such examples are aluminium carbide Al
4C
3, magnesium carbide Mg
2C^[3] and beryllium carbide Be
2C.

Transition metal carbides are not saline carbides but 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.^[4]

Note that methanide in this context is a trivial historical name, according to IUPAC systematic naming conventions a compound such as NaCH₃ would be termed a "methanide", although this compound is often called methylsodium.^[5]

Acetylides

Calcium carbide.

Several carbides are assumed to be salts of the acetylide anion C₂^2– (also called percarbide), which has a triple bond between the two carbon atoms. Alkali metals, alkaline earth metals, and lanthanoid metals form acetylides, e.g., sodium carbide Na₂C₂, calcium carbide CaC₂, and LaC₂.^[1] 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 C₂²⁻.

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 C₂²⁻, explaining the metallic conduction.^[1]

Sesquicarbides

The polyatomic ion C₃⁴⁻, sometimes called sesquicarbide or allylenide, is found in Li₄C₃ and Mg₂C₃. The ion is linear and is isoelectronic with CO₂.^[1] The C-C distance in Mg₂C₃ is 133.2 pm.^[6] Mg₂C₃ yields methylacetylene, CH₃CCH, and propadiene, CH₂CCH₂, on hydrolysis, which was the first indication that it contains C₃⁴⁻.

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.^[1] 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, e.g. B₂₅C.

Interstitial carbides

The carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described as interstitial compounds.^[1] These carbides 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.^[7]

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:^[1]

• 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.
• 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.

The following table^[1][7] shows actual 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.

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.^[8]

Intermediate transition metal carbides

In these carbides, 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 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.^[1]

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⁺ 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, e.g., CRuCl₂(P(C₆H₁₁)₃)₂.

Metallocarbohedrynes (or "met-cars") are stable clusters with the general formula M
8C
12 where M is a transition metal (Ti, Zr, V, etc.).

Impossible carbides

Some metals, such as lead and tin, are believed not to form carbides under any circumstances.^[9] There exists however a mixed titanium-tin carbide, which is a two-dimensional conductor.^[10] (In 2007, there were two reports of a lead carbide PbC₂, apparently of the acetylide type; but these claims have yet to be published in reviewed journals.)

Related materials

In addition to the carbides, other groups of related carbon compounds exist:^[1]

• 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

References