In chemistry, a carbide usually describes a binary phase composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece.
Interstitial / Metallic carbides
The carbides of the group 4, 5 and 6 transition metals (with the exception of chromium) are often described as interstitial compounds.
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.
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:
-
When the metal atoms are close-packing, (ccp), then filling all of the octahedral interstices with carbon achieves 1:1 stoichiometry with the rock salt structure.
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When the metal atoms are close-packing, (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.
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The following table
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.
Iron forms a number of carbides, , and . 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.
Some metals, such as lead and tin, are believed not to form carbides under any circumstances.
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 (CaC
2),
silicon carbide (SiC),
tungsten carbide (WC; often called, simply,
carbide when referring to machine tooling), and
cementite (Fe
3C),
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
, alkaline earth metals,
,
, 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 "C
4−", in the methanides or methides; two-atom units, "", in the
; and three-atom units, "", in the allylides.
The graphite intercalation compound KC
8, prepared from vapour of potassium and graphite, and the alkali metal derivatives of C
60 are not usually classified as carbides.
[Shriver and Atkins — Inorganic Chemistry]
Methanides
Methanides are a subset of carbides distinguished by their tendency to decompose in water producing
methane. Three examples are aluminium carbide , magnesium carbide
and beryllium carbide .
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.
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.
Acetylides/ethynides
Several carbides are assumed to be salts of the
acetylide (also called percarbide, by analogy with
peroxide), which has a
covalent bond between the two carbon atoms. Alkali metals, alkaline earth metals, and
lanthanoid form acetylides, for example,
sodium carbide Na
2C
2,
calcium carbide CaC
2, and LaC
2.
Lanthanides also form carbides (sesquicarbides, see below) with formula M
2C
3. Metals from group 11 also tend to form acetylides, such as copper(I) acetylide and
silver acetylide. Carbides of the
actinides, which have stoichiometry MC
2 and M
2C
3, are also described as salt-like derivatives of .
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 uranium carbide. The bonding in LaC2 has been described in terms of LaIII with the extra electron delocalised into the antibonding orbital on , explaining the metallic conduction.
Allylides
The
polyatomic ion , sometimes called
allylide, is found in and . The ion is linear and is
isoelectronic with .
The C–C distance in is 133.2 pm.
yields
methylacetylene, , and
propadiene, , on hydrolysis, which was the first indication that it contains .
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.
, B
4C, 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
. 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
25C.
Molecular carbides
Metal complexes containing C are known as metal carbido complexes. Most common are carbon-centered octahedral clusters, such as (where "Ph" represents a
phenyl group) and . Similar species are known for the
and the early metal halides. A few terminal carbides have been isolated, such as .
Metallocarbohedrynes (or "met-cars") are stable clusters with the general formula where M is a transition metal (Ti, Zr, V, etc.).
Related materials
In addition to the carbides, other groups of related carbon compounds exist:
-
graphite intercalation compounds
-
alkali metal fullerides
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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
