# 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 $_x$O$^{n−}$ $_y$ for some integers x, y, and n.
The most common oxocarbon anions are carbonate, CO2−3, and oxalate, C$_2$O2−4. 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 CO$^{4−}$, 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 H$_3$C−O−(C=O)$_2$−O−CH$_3$.

## Electronic structure of the carbonate ion

The carbonate ion has a trigonal planar structure, point group D$_{3h}$. The three C-O bonds have the same length of 136 pm and the 3 O-C-O angles are 120°. The carbon atom has 4 pairs of valence electrons, which shows that the molecule obeys the octet rule. This is one factor that contributes to the high stability of the ion, which occurs in rocks such as limestone. The electronic structure is described by two main theories which are used to show how the 4 electron pairs are distributed in a molecule that only has 3 C-O bonds.
With valence bond theory the electronic structure of the carbonate ion is a resonance hybrid of 3 canonical forms.
In each canonical form there are two single bonds one double bond. The three canonical forms contribute equally to the resonance hybrid, so the three bond C-O bonds have the same length.
With molecular orbital theory the 3-fold axis is designated as the z axis of the molecule. Three σ bonds are formed overlap of the s, p$_x$ and p$_y$ orbitals on the carbon atom with a p orbital on each oxygen atom. In addition, a delocalized π bond is made by overlap of the p$_z$ orbital on the carbon atom with the p$_z$ orbital on each oxygen atom which is perpendicular to the plane of the molecule.
Note that the same bonding schemes may be applied the nitrate ion, NO$_3$$^−$, which is isoelectronic with the carbonate ion.
Similarly, the two-fold symmetrical structure of a carboxylate group,CO$^–$ $_2$, may be described as a resonance hybrid of two canonical forms in valence bond theory, or with 2 σ bonds and a delocalized π bond in molecular orbital theory.

## Related compounds
### Oxocarbon acids

An oxocarbon anion C $_x$O$^{n−}$ $_y$ can be seen as the result of removing all protons from a corresponding acid C$_x$H$_n$O$_y$. Carbonate CO$^{2−}$ $_3$, for example, can be seen as the anion of carbonic acid H$_2$CO$_3$. Sometimes the "acid" is actually an alcohol or other species; this is the case, for example, of acetylenediolate C $_2$O$^{2−}$ $_2$ that would yield acetylenediol C$_2$H$_2$O$_2$. However, the anion is often more stable than the acid (as is the case for carbonate); and sometimes the acid is unknown or is expected to be extremely unstable (as is the case of methanetetracarboxylate C(COO$^−$)$_4$).

### Neutralized species

Every oxocarbon anion C $_x$O$^{n−}$ $_y$ can be matched in principle to the electrically neutral (or oxidized) variant C$_x$O$_y$, an oxocarbon (oxide of carbon) 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 CO$_3$; oxalate C $_2$O$^{2−}$ $_4$ correspond to the even less stable 1,2-dioxetanedione C$_2$O$_4$; and the stable croconate anion C $_5$O$^{2−}$ $_5$ corresponds to the neutral cyclopentanepentone C$_5$O$_5$, 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 C $_6$O$^{2−}$ $_6$ can be reduced to the tetrahydroxybenzoquinone (THBQ) anion C $_6$O$^{4−}$ $_6$ and then to benzenehexolate C $_6$O$^{6−}$ $_6$.

### Acid anhydrides

An oxocarbon anion C $_x$O$^{n−}$ $_y$ can also be associated with the anhydride of the corresponding acid. The latter would be another oxocarbon with formula C$_x$O$_{y−n⁄2}$; namely, the acid minus n⁄2 water molecules H$_2$O. The standard example is the connection between carbonate CO$^{2−}$ $_3$ and carbon dioxide CO$_2$. 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 C$_{12}$O$_9$ from mellitate C $_{12}$O$^{6−}$ $_{12}$ via mellitic acid C$_{12}$H$_6$O$_{12}$

### Hydrogenated anions

For each oxocarbon anion C $_x$O$^{n−}$ $_y$ there are in principle n−1 partially hydrogenated anions with formulas H $_k$C $_x$O$^{(n−k)−}$ $_y$, 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 HCO$^−$ $_3$ is commonly called bicarbonate, and hydrogenoxalate HC $_2$O$^−$ $_4$ is known as binoxalate.
The hydrogenated anions may be stable even if the fully protonated acid is not (as is the case of bicarbonate).

## List of oxocarbon anions

Here is an incomplete list of the known or conjectured oxocarbon anions
| Formula | Name | Acid | Anhydride | Neutralized |
| :CO$^{2−}$ $_2$ | carbonite | C(OH)$_2$ (carbonous acid) | CO | CO$_2$ |
| CO$^{2−}$ $_3$ | carbonate | CH$_2$O$_3$ | CO$_2$ | CO$_3$ |
| CO$^{2−}$ $_4$ | peroxocarbonate | | CO$_3$ | CO$_4$ |
| CO$^{4−}$ $_4$ | orthocarbonate | C(OH)$_4$ methanetetrol | CO$_2$ | CO$_4$ |
| C $_2$O$^{2−}$ $_2$ | acetylenediolate | C$_2$H$_2$O$_2$ acetylenediol | | C$_2$O$_2$ |
| C $_2$O$^{2−}$ $_4$ | oxalate | C$_2$H$_2$O$_4$ | C$_2$O$_3$, C$_4$O$_6$ | C$_2$O$_4$ |
| C $_2$O$^{2−}$ $_5$ | dicarbonate | C$_2$H$_2$O$_5$ | C$_2$O$_4$ | |
| C $_2$O$^{2−}$ $_6$ | peroxodicarbonate | | | |
| C $_3$O$^{2−}$ $_3$ | deltate | C$_3$O(OH)$_2$ | | C$_3$O$_3$ |
| C $_3$O$^{2−}$ $_5$ | mesoxalate | C$_3$H$_2$O$_5$ | | |
| C $_4$O$^{2−}$ $_4$ | acetylenedicarboxylate | C$_4$H$_2$O$_4$ | | |
| C $_4$O$^{2−}$ $_4$ | squarate | C$_4$O$_2$(OH)$_2$ | | C$_4$O$_4$ |
| C $_4$O$^{2−}$ $_6$ | dioxosuccinate | C$_4$H$_2$O$_6$ | | |
| C $_5$O$^{2−}$ $_5$ | croconate | C$_5$O$_3$(OH)$_2$ | | C$_5$O$_5$ |
| C $_5$O$^{4−}$ $_8$ | methanetetracarboxylate | C$_5$H$_4$O$_8$ | | |
| C $_6$O$^{2−}$ $_6$ | rhodizonate | C$_4$O$_4$(COH)$_2$ | | C$_6$O$_6$ |
| C $_6$O$^{4−}$ $_6$ | benzoquinonetetraolate; THBQ anion | (CO)$_2$(COH)$_4$ THBQ | | C$_6$O$_6$ |
| C $_6$O$^{6−}$ $_6$ | benzenehexolate | C$_6$(OH)$_6$ benzenehexol | | C$_6$O$_6$ |
| C $_6$O$^{4−}$ $_8$ | ethylenetetracarboxylate | C$_6$H$_4$O$_8$ | C$_6$O$_6$ | |
| C $_8$O$^{4−}$ $_9$ | furantetracarboxylate | C$_8$H$_4$O$_9$ | C$_8$O$_7$ | |
| C $_{10}$O$^{4−}$ $_{10}$ | benzoquinonetetracarboxylate | C $_{10}$H $_4$O $_{10}$ | C $_{10}$O $_8$ | |
| C $_{12}$O$^{6−}$ $_{12}$ | mellitate | C$_6$(COOH)$_6$ | C$_{12}$O$_9$ | |
Several other oxocarbon anions have been detected in trace amounts, such as C $_6$O$^−$ $_6$, a singly ionized version of rhodizonate.