Oxocarbon anion

In chemistry, an oxocarbon anion is a negative ion consisting solely of carbon and oxygen atoms, and therefore having the general formula C
_xOⁿ⁻
_y for some integers x, y, and n.

The most common oxocarbon anions are carbonate, CO2−3, and oxalate, C₂O2−4. There are however a large number of stable anions in this class, including several ones that have research or industrial use. There are also many unstable anions, like CO−2 and CO⁴⁻, that have a fleeting existence during some chemical reactions; and many hypothetical species, like CO4−4, that have been the subject of theoretical studies but have yet to be observed.

Stable oxocarbon anions form salts with a large variety of cations. Unstable anions may persist in very rarefied gaseous state, such as in interstellar clouds. Most oxocarbon anions have corresponding moieties in organic chemistry, whose compounds are usually esters. Thus, for example, the oxalate moiety [−O−(C=O)₂−O−] occurs in the ester dimethyl oxalate H₃C−O−(C=O)₂−O−CH₃.