In chemistry, an oxocarbon anion is a negative ion consisting solely of carbon and oxygen atoms, and therefore having the general formula CxOn−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
The carbonate ion has a trigonal planar structure, point group D3h. 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.
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, px, and py 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 pz orbital on the carbon atom with the pz 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 CxOn−y can be seen as the result of removing all protons from a corresponding acid CxHnOy. 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 CxOn−y can be matched in principle to the electrically neutral (or oxidized) variant CxOy, 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 C2O2−4 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 CxOn−y can sometimes be associated with the anhydride of the corresponding acid. The latter would be an oxocarbon with formula CxOy−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 CxOn−y there are in principle n−1 partially hydrogenated anions with formulas HkCxO(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.
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
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