A compound formed by linking two pyridine rings by a single C-C bond, (C5H4N)2. Various isomers are possible depending on the relative positions of the nitrogen atoms. A mixture of isomers can be made by reacting pyridine with sodium metal and oxidizing the resulting sodium salt. The 22′-isomer is a powerful chelating agent denoted bipy in chemical formulae. Both the 22′- and 44′-isomers can form quaternary compounds that are used in herbicides such as Paraquat (44′) and Diquat (22′).