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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 Methanides contain the monatomic ion C4−. Examples of methanides are Be2C and Al4C3. The C4− ion is a very strong base and will combine with four protons to form methane. The reaction may be written as follows: C4− + 4H+ → CH4 Methanides commonly react with water to form methane, but reactions with other substances are also common.