Oxocarbon anion

2D diagram of mellitate C
₁₂O⁶⁻
₁₂, one of the oxocarbon anions. Black circles are carbon atoms, red circles are oxygen atoms. Each blue halo represents one half of a negative charge.

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, C
₂O²⁻
₄. 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 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 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:

so that each oxygen has the same negative charge equivalent to 2⁄3 of an 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 –CO^–
₂, 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 C
₅O²⁻
₅ 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.

Related compounds

Oxocarbon acids

An oxocarbon anion C
_xOⁿ⁻
_y can be seen as the result of removing all protons from a corresponding acid C_xH_nO_y. Carbonate CO²⁻
₃, 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 C
₂O²⁻
₂ that would yield acetylenediol C₂H₂O₂. 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
_xOⁿ⁻
_y can be matched in principle to the electrically neutral (or oxidized) variant C_xO_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 CO₃; oxalate C
₂O²⁻
₄ correspond to the even less stable 1,2-dioxetanedione C₂O₄; and the stable croconate anion C
₅O²⁻
₅ 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 C
₆O²⁻
₆ can be reduced to the tetrahydroxybenzoquinone (THBQ) anion C
₆O⁴⁻
₆ and then to benzenehexolate C
₆O⁶⁻
₆.

Acid anhydrides

An oxocarbon anion C
_xOⁿ⁻
_y can also be associated with the anhydride of the corresponding acid. The latter would be another oxocarbon with formula C_xO_y−n⁄2; namely, the acid minus n⁄2 water molecules H₂O. The standard example is the connection between carbonate CO²⁻
₃ 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 C
₁₂O⁶⁻
₁₂ via mellitic acid C₁₂H₆O₁₂

Hydrogenated anions

For each oxocarbon anion C
_xOⁿ⁻
_y there are in principle n−1 partially hydrogenated anions with formulas H
_kC
_xO^(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
₂O⁻
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).

Extreme cases

The carbide anions, such as acetylide C²⁻
₂ and methanide C⁴⁻, could be seen as extreme cases of oxocarbon anions C
_xOⁿ⁻
_y, with y equal to zero. The same could be said of oxygen-only anions such as oxide O²⁻, superoxide O⁻
₂, peroxide, O²⁻
₂, and ozonide O⁻
₃.

List of oxocarbon anions

Here is an incomplete list of the known or conjectured oxocarbon anions

Diagram	Formula	Name	Acid	Anhydride	Neutralized
	:CO2− 2	carbonite	C(OH)2	CO	CO2
	CO2− 3	carbonate	CH2O3	CO2	CO3
	CO2− 4	peroxocarbonate		CO3	CO4
	CO4− 4	orthocarbonate	C(OH)4 methanetetrol	CO2	CO4
	C 2O2− 2	acetylenediolate	C2H2O2 acetylenediol		C2O2
	C 2O2− 4	oxalate	C2H2O4	C2O3, C4O6	C2O4
	C 2O2− 5	dicarbonate	C2H2O5	C2O4
	C 2O2− 6	peroxodicarbonate
	C 3O2− 3	deltate	C3O(OH)2		C3O3
	C 3O2− 5	mesoxalate	C3H2O5
	C 4O2− 4	acetylenedicarboxylate	C4H2O4
	C 4O2− 4	squarate	C4O2(OH)2		C4O4
	C 4O2− 6	dioxosuccinate	C4H2O6
	C 5O2− 5	croconate	C5O3(OH)2		C5O5
	C 5O4− 8	methanetetracarboxylate	C5H4O8
	C 6O2− 6	rhodizonate	C4O4(COH)2		C6O6
	C 6O4− 6	benzoquinonetetraolate; THBQ anion	(CO)2(COH)4 THBQ		C6O6
	C 6O6− 6	benzenehexolate	C6(OH)6 benzenehexol		C6O6
	C 6O4− 8	ethylenetetracarboxylate	C6H4O8	C6O6
	C 8O4− 9	furantetracarboxylate	C8H4O9
	C 10O4− 10	benzoquinonetetracarboxylate	C 10H 4O 10	C 10O 8
	C 12O6− 12	mellitate	C6(COOH)6	C12O3

Several other oxocarbon anions have been detected in trace amounts, such as C
₆O⁻
₆, a singly ionized version of rhodizonate.

See also

• Oxocarbon
• Silicate
• Sodium percarbonate (actually a carbonate perhydrate)