Oxocarbon anion


In chemistry, an oxocarbon anion is a negative ion consisting solely of carbon and oxygen atoms, and therefore having the general formula for some integers x, y, and n.
The most common oxocarbon anions are carbonate,, and oxalate,. 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 and, that have a fleeting existence during some chemical reactions; and many hypothetical species, like Orthocarbonic acid|, 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–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:

so that each oxygen has the same negative charge equivalent to of an electron, and each C–O bond has the same average valence of. 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 − and each C–O bond to have valence, 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 − on each oxygen. These resonances are believed to contribute to the stability of the anions.

Related compounds

Oxocarbon acids

An oxocarbon anion can be seen as the result of removing all protons from a corresponding acid CxHnOy. Carbonate, for example, can be seen as the anion of carbonic acid H2CO3. Sometimes the "acid" is actually an alcohol or other species; this is the case, for example, of acetylenediolate that would yield acetylenediol C2H2O2. However, the anion is often more stable than the acid ; and sometimes the acid is unknown or is expected to be extremely unstable.

Neutralized species

Every oxocarbon anion can be matched in principle to the electrically neutral variant CxOy, an oxocarbon with the same composition and structure except for the negative charge. As a rule, however, these neutral oxocarbons are less stable than the corresponding anions. Thus, for example, the stable carbonate anion corresponds to the extremely unstable neutral carbon trioxide CO3; oxalate correspond to the even less stable 1,2-dioxetanedione C2O4; and the stable croconate anion corresponds to the neutral cyclopentanepentone C5O5, which has been detected only in trace amounts.

Reduced variants

Conversely, some oxocarbon anions can be reduced to yield other anions with the same structural formula but greater negative charge. Thus rhodizonate can be reduced to the tetrahydroxybenzoquinone anion and then to benzenehexolate.

Acid anhydrides

An oxocarbon anion can also be associated with the anhydride of the corresponding acid. The latter would be another oxocarbon with formula CxOy−; namely, the acid minus water molecules H2O. The standard example is the connection between carbonate and carbon dioxide CO2. The correspondence is not always well-defined since there may be several ways of performing this formal dehydration, including joining two or more anions to make an oligomer or polymer. Unlike neutralization, this formal dehydration sometimes yields fairly stable oxocarbons, such as mellitic anhydride C12O9 from mellitate via mellitic acid C12H6O12

Hydrogenated anions

For each oxocarbon anion there are in principle n−1 partially hydrogenated anions with formulas, where k ranges from 1 to n−1. These anions are generally indicated by the prefixes "hydrogen"-, "dihydrogen"-, "trihydrogen"-, etc. Some of them, however, have special names: hydrogencarbonate is commonly called bicarbonate, and hydrogenoxalate is known as binoxalate.
The hydrogenated anions may be stable even if the fully protonated acid is not.

Extreme cases

The carbide anions, such as acetylide and methanide C4−, could be seen as extreme cases of oxocarbon anions, with y equal to zero. The same could be said of oxygen-only anions such as oxide O2−, superoxide, peroxide,, and ozonide.

List of oxocarbon anions

Here is an incomplete list of the known or conjectured oxocarbon anions
DiagramFormulaNameAcidAnhydrideNeutralized
:carboniteHCO2HCOCO2
carbonateCH2O3CO2CO3
peroxocarbonateCO3CO4
orthocarbonateC4 methanetetrolCO2CO4
acetylenediolateC2H2O2 acetylenediolC2O2
oxalateC2H2O4C2O3, C4O6C2O4
dicarbonateC2H2O5C2O4
peroxodicarbonate
deltateC3O2C3O3
mesoxalateC3H2O5
acetylenedicarboxylateC4H2O4
squarateC4O22C4O4
dioxosuccinateC4H2O6
croconateC5O32C5O5
methanetetracarboxylateC5H4O8
rhodizonateC4O42C6O6
benzoquinonetetraolate; THBQ anion24 THBQC6O6
benzenehexolateC66 benzenehexolC6O6
ethylenetetracarboxylateC6H4O8C6O6
furantetracarboxylateC8H4O9
benzoquinonetetracarboxylatebenzoquinonetetracarboxylic acid|benzoquinonetetracarboxylic dianhydride|
mellitateC66C12O9

Several other oxocarbon anions have been detected in trace amounts, such as, a singly ionized version of rhodizonate.