Salt-like materials

Salt-like carbides are composed of highly electropositive elements such as the the alkali metals, alkaline earths, and group 3 metals including (scandium, yttrium and lanthanum.

The naming of ionic carbides is not consistent and can be quite confusing.

Acetylides

The hypothetical polyatomic ion C₂²⁻ is described by a triple bond between the two carbon atoms. Alkali and alkaline earth metals form acetylides, e.g., Na₂C₂ and CaC₂. Lanthanoids are similar, 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 C₂^2−, explaining the metallic conduction. Metals from group 11 tend to form acetylides, such as copper(I) acetylide and silver acetylide. lanthanides when forming MC₂ and M₂C₃ carbides. Actinides, which form materials of the stoichiometry MC₂ and M₂C₃, are described as salt-like derivatives of C₂²⁻, although percarbides are also known.

Methanides

The monatomic ion C⁴⁻ would be highly basic, but is unknown except in under extreme conditions. Methanides in principle would react with water to form methane.

C⁴⁻ + 4H⁺ → CH₄

Examples of compounds that are described as methanides include C⁴⁻ are Be₂C and Al₄C₃.

Sesquicarbides

The polyatomic ion C₃⁴⁻ is found in 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⁻. The ion C₃⁴⁻ is sometimes called sesquicarbides, they hydrolyse to give methylacetylene.

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. 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 industrially. Boron also forms other covalent carbides, e.g. B₂₅C.