In chemistry, a carbide is a compound of carbon with a less electronegative element. Many carbides are important industrially; for example, calcium carbide (CaC2) is a feedstock for the chemical industry and iron carbide (Fe3C, cementite) is formed in steels to improve their properties.
In general, carbides are classified according to the chemical bonding in the compounds, as follows:
Salt-like, ionic carbides are formed by the following metals:
Most commonly, they are salts of C22− and are called acetylides, ethynides, acetylenediides, or (rarely) percarbides.
Some ionic carbides contain other anionic species, such as:
The naming of ionic carbides is not consistent and can be quite confusing.
Acetylides contain the polyatomic ion C22−, in which there is a triple bond between the two carbon atoms (similar to acetylene). Examples are carbides of the alkali metals (such as Na2C2), some alkaline earths (such as CaC2) and lanthanoids (such as LaC2).
The C-C bond distance ranges from 109.2 picometers (pm) in CaC2 (similar to acetylene), to 130.3 pm in LaC2 and 134pm in UC2.
Methanides commonly react with water to form methane, but reactions with other substances are also common.
The polyatomic ion C34− is found in, for instance, Li4C3 and Mg2C3. The ion is linear and isoelectronic with CO2. The C-C distance in Mg2C3 is 133.2 pm. Hydrolysis of Mg2C3 yields methylacetylene (CH3CCH), which was the first indication that it may contain C34−.
Silicon and boron form covalent carbides. Silicon carbide (SiC, or carborundum) has two similar crystalline forms, both of which are related to the structure of diamond. 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 and boron carbide are very hard, refractory (heat-resistant) materials. Both are important industrially. Boron also forms other covalent carbides, such as B25C.
The carbides of transition metals of groups four, five, and six (with the exception of chromium) are often described as interstitial compounds. They are chemically quite inert, have metallic properties, and are refractory. Some exhibit a range of stoichiometries (that is, the metal and carbon atoms can be combined in different proportions). Titanium carbide and tungsten carbide are important industrially and are used to coat metals in cutting tools.
The carbon atoms are thought to fit into octahedral interstices in the metal lattice, when the metal atom radius is greater than 135 pm. If the metal atoms are cubic close packed (face-centered cubic), then eventually all the interstices could be filled to give a 1:1 stoichiometry, with the rock salt structure, as in the case of tungsten carbide (WC). When the metal atoms are hexagonal close packed, then only half the interstices are filled, giving a stoichiometry of 2:1, as in the case of divanadium carbide (V2C).
The following table shows actual structures of metals and their carbides. The notation "h/2" refers to the V2C-type structure mentioned above, which is an approximate description of the actual structures. The simple view that the lattice of the pure metal "absorbs" carbon atoms is true only for the monocarbides of vanadium (VC) and niobium (NbC).
|Metal||Structure||Metallic radius (pm)||MC structure||M2C structure||Other carbides|
|vanadium||cubic body centered||134||rock salt||h/2||V4C3|
|niobium||cubic body centered||146||rock salt||h/2||Nb4C3|
|tantalum||cubic body centered||146||rock salt||h/2||Ta4C3|
|chromium||cubic body centered||128||Cr23C6, Cr3C, Cr7C3, Cr3C2|
|molybdenum||cubic body centered||139||hexagonal||h/2||Mo3C2|
|tungsten||cubic body centered||139||hexagonal||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.
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 of these 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 hydrolyzed 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.
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