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, NO₃⁻, 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.

Oxocarbon acids edit

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);^[1] and sometimes the acid is unknown or is expected to be extremely unstable (as is the case of methanetetracarboxylate C(COO⁻)₄).

Neutralized species edit

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;^[2] oxalate C
₂O²⁻
₄ correspond to the even less stable 1,2-dioxetanedione C₂O₄;^[3] and the stable croconate anion C
₅O²⁻
₅ corresponds to the neutral cyclopentanepentone C₅O₅, which has been detected only in trace amounts.^[4]

Reduced variants edit

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.^[5]

Acid anhydrides edit

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^[6][7][8]

Hydrogenated anions edit

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).

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

Several other oxocarbon anions have been detected in trace amounts, such as C
₆O⁻
₆, a singly ionized version of rhodizonate.^[9]

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