Oxocarbon anion

2D diagram of mellitate C₁₂O6−12, 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, C₂O2−4. There are, however, a large number of stable anions in this class, including several 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.

Electronic structure of the carbonate ion

Space-filling model 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 C–O bonds have the same length.

Making a π-bond between 2 atoms of the same chemical element

With molecular orbital theory, the 3-fold axis is designated as the z-axis of the molecule. Three σ bonds are formed by the 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.

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
_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 C₂O2−2 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 corresponds to the even-less-stable 1,2-dioxetanedione C₂O₄; and the stable croconate anion C₅O2−5 corresponds to the neutral cyclopentanepentone C₅O₅ (once called leuconic acid), 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 C₆O2−6 can be reduced to the tetrahydroxybenzoquinone (THBQ) anion C₆O4−6 and then to benzenehexolate C₆O6−6. In this specific case, there exists a stable radical anion C₆O3−6 which is formed by simultaneous oxidation of C₆O4−6 and reduction of C₆O2−6 in an electrolytic cell.

Acid anhydrides

An oxocarbon anion C
_xOⁿ⁻
_y can sometimes be associated with the anhydride of the corresponding acid. The latter would be an 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 C₁₂O6−12 via mellitic acid C₁₂H₆O₁₂, or benzoquinonetetracarboxylic dianhydride C₁₀O₈ from the corresponding acid C₁₀H₄O₁₀ and anion C₁₀O4−10.

A notable exception to the general stability of these anhydrides is dioxalic anhydride or dioxane tetraketone (C₄O₆), which decomposes above 0 °C, although the oxalate anion C₂O2−4 is ubiquitous. The highly unstable carbonite anion has no directly associated acid (formic acid H(CO)OH is an isomer of the hypothetical carbonous acid C(OH)₂) and thus cannot be associated with its "anhydride" carbon monoxide, which does not act as a practical precursor to any oxocarbonic acid.

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 HC₂O−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.

Diagram	Formula	Name	Acid	Anhydride	Neutralized
	:CO2−2	carbonite	C(OH)2 (carbonous acid)	CO	CO2
	CO2−3	carbonate	CH2O3	CO2	CO3
	CO2−4	peroxocarbonate		CO3	CO4
	CO4−4	orthocarbonate	C(OH)4 (methanetetrol)	CO2	CO4
	C2O2−2	acetylenediolate	C2H2O2 (acetylenediol)		C2O2
	C2O2−4	oxalate	C2H2O4	C2O3, C4O6	C2O4
	C2O2−5	dicarbonate	C2H2O5	C2O4
	C2O2−6	peroxodicarbonate
	C3O2−3	deltate	C3O(OH)2		C3O3
	C3O2−5	mesoxalate	C3H2O5
	C4O2−4	acetylenedicarboxylate	C4H2O4
	C4O2−4	squarate	C4O2(OH)2		C4O4
	C4O2−6	dioxosuccinate	C4H2O6
	C5O2−5	croconate	C5O3(OH)2		C5O5
	C5O4−8	methanetetracarboxylate	C5H4O8
	C6O2−6	rhodizonate	C4O4(COH)2		C6O6
	C6O4−6	benzoquinonetetraolate; THBQ anion	(CO)2(COH)4 TBHQ		C6O6
	C6O6−6	benzenehexolate	C6(OH)6 benzenehexol		C6O6
	C6O4−8	ethylenetetracarboxylate	C6H4O8	C6O6
	C8O4−9	furantetracarboxylate	C8H4O9	C8O7
	C10O4−10	benzoquinonetetracarboxylate	C10H4O10	C10O8
	C12O6−12	mellitate	C6(COOH)6	C12O9

Several other oxocarbon anions have been detected in trace amounts, such as C₆O−6, a singly-ionized version of rhodizonate.

See also

• Oxocarbon
• Silicate
• Sodium percarbonate (actually a carbonate perhydrate)
• Pseudo-oxocarbon anion