A cyclooctadiene (sometimes abbreviated COD) is any of several cyclic diene with the formula (CH₂)₄(C₂H₂)₂. Focusing only on cis derivatives, four isomers are possible: 1,2-, which is an allene, 1,3-, 1,4-, and 1,5-. Commonly encountered isomers are the conjugated isomer 1,3-cyclooctadiene and 1,5-cyclooctadiene, which is used as a ligand for transition metals. These dienes are colorless volatile liquids.^[1]