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In chemistry, an oxocarbon anion is a negative ion consisting solely of carbon and oxygen atoms, and therefore having the general formula C''x''O''y''''n''− for some integers ''x'', ''y'', and ''n''. The most common oxocarbon anions are carbonate, CO32−, and oxalate, C2O42−. There is 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 CO2− and CO4−, that have a fleeting existence during some chemical reactions; and many hypothetical species, like CO44−, 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 () occurs in the ester dimethyl oxalate H3C–O–(C=O–)2–O–CH3. == Distributed charges and resonances == In many oxocarbon anions each of the extra electrons responsible for the negative electric charges behaves as if it were distributed over several atoms. Some of the electron pairs responsible for the covalent bonds also behave as if they were delocalized. These phenomena are often explained as a resonance between two or more conventional molecular structures that differ on the location of those charges and bonds. The carbonate ion, for example, is considered to have an "average" of three different structures 400px so that each oxygen has the same negative charge equivalent to 2/3 of one electron, and each C–O bond has the same average valence of 4/3. This model accounts for the observed threefold symmetry of the anion. Similarly, in a deprotonated carboxyl group –, each oxygen is often assumed to have a charge of −1/2 and each C–O bond to have valence 3/2, so the two oxygens are equivalent. The croconate anion also has fivefold symmetry, that can be explained as the superposition of five states leading to a charge of −2/5 on each oxygen. These resonances are believed to contribute to the stability of the anions. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Oxocarbon anion」の詳細全文を読む スポンサード リンク
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