Ethylene dione or ethylenedione, also called dicarbon dioxide, ethenedione, or ethene 1,2-dione, is the name given to a hypothetical chemical compound with the formula C2O2 or O=C=C=O. It is a dimer of carbon monoxide and belongs to the oxocarbon series. It can be thought of as ketene of glyoxylic acid (OHCCOOH). Despite many attempts to prepare it, the compound has never been isolated in substance or been observed as a transient species.
Molecular orbital considerations indicate that the ground state of linear ethenedione is a triplet, and that dissociation should be slow, because triplet carbon monoxide is a high-energy particle. However, when the molecule is distorted away from its equilibrium geometry, the potential surfaces of the triplet and singlet states intersect, allowing for intersystem crossing to the singlet state, which is unbound and dissociates on a timescale of 0.5 ns.
In the 1940s, Detroit physician William Frederick Koch claimed that he had synthesized this compound, which he called glyoxylide, and that it was an antidote to the toxins that caused a long list of ailments, including diabetes and cancer. Neither claim was ever confirmed, and the drug was classified as a fraud by the FDA.