.]] 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, silicon carbide, tungsten carbide (often called simply carbide), and cementite, each used in key industrial applications.
Methanides in general chemical context refers to any compound that hydrolyzes to methane, which might include also salts with hydrogenated anions such as , , and . However, according to IUPAC systematic naming conventions, only the last is properly called "methanide". In theory one can describe compounds that contain the methyl group, with relatively large bond polarity between the carbon and non-hydrogen atom, as salts of this anion; however in truth most such compounds, if not all, are, in fact covalent.
The C-C triple bond length ranges from 109.2 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 LaIII with the extra electron delocalised into the antibonding orbital on C22
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:
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, 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 | 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.
* Category:Anions Category:Salts
bg:Карбид cs:Karbidy da:Karbid de:Carbide es:Carburo eo:Karbido fr:Carbure id:Karbida it:Carburi he:קרביד kk:Карбидтер lv:Karbīdi hu:Karbid nl:Carbide ja:炭化物 no:Karbid pl:Węgliki pt:Carbeto ru:Карбиды sl:Karbid fi:Karbidi sv:Karbid uk:Карбіди vi:Cacbua zh:碳化物This text is licensed under the Creative Commons CC-BY-SA License. This text was originally published on Wikipedia and was developed by the Wikipedia community.
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