Halogens, elemental acetylenes

Insertion Reactions into Element-Halogen Bonds 11.6.2. Insertions of Acetylenes and Olefins... 

Carbon, like several other elements, exhibits a variable valency in its compounds. Thus it is tetravalent in the greater number of organic compounds, trivalent in triphenylmethyl, and pentaphenylethyl, etc., and divalent in carbon monoxide, fulminic acid and its derivatives, in the halogenated compounds of acetylene, etc. 

Functional groups can contain many elements, but the most common are oxygen and nitrogen. Sulfur and the halogens are less commonly encountered. Some functional groups are part of the molecular backbone. These include the multiple bonds between backbone carbon atoms of compounds such as ethene (ethylene), ethyne (acetylene), and benzene. 

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