In
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
, an oxocarbon anion is a
negative ion consisting solely of
carbon
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
and
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
atoms, and therefore having the general formula for some integers ''x'', ''y'', and ''n''.
The most common oxocarbon anions are
carbonate
A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...
, , and
oxalate
Oxalate (systematic IUPAC name: ethanedioate) is an anion with the chemical formula . This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (), and several esters such as ...
, . 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 and , that have a fleeting existence during some chemical reactions; and many hypothetical species, like
, that have been the subject of theoretical studies but have yet to be observed.
Stable oxocarbon anions form
salts
In chemistry, a salt or ionic compound is a chemical compound consisting of an assembly of positively charged ions ( cations) and negatively charged ions (anions), which results in a compound with no net electric charge (electrically neutral). ...
with a large variety of
cation
An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s. Unstable anions may persist in very rarefied gaseous state, such as in
interstellar cloud
An interstellar cloud is an accumulation of gas, plasma, and cosmic dust in galaxies. Put differently, an interstellar cloud is a denser-than-average region of the interstellar medium, the matter and radiation that exists in the space between ...
s. Most oxocarbon anions have corresponding
moieties in
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
, whose compounds are usually
esters
In chemistry, an ester is a chemical compound, compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds c ...
. Thus, for example, the oxalate moiety occurs in the ester
dimethyl oxalate
Dimethyl oxalate is an organic compound with the formula or . It is the dimethyl ester of oxalic acid. Dimethyl oxalate is a colorless or white solid that is soluble in water.
Production
Dimethyl oxalate can be obtained by esterification of oxa ...
.
Electronic structure of the carbonate ion

The carbonate ion has a trigonal planar structure,
point group
In geometry, a point group is a group (mathematics), mathematical group of symmetry operations (isometry, isometries in a Euclidean space) that have a Fixed point (mathematics), fixed point in common. The Origin (mathematics), coordinate origin o ...
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
The octet rule is a chemical rule of thumb that reflects the theory that main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, giving it the same electronic configuration as a noble gas. The ru ...
. This is one factor that contributes to the high stability of the ion, which occurs in rocks such as
limestone
Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
. 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
In chemistry, valence bond (VB) theory is one of the two basic theories, along with molecular orbital (MO) theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of ...
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
In chemistry, molecular orbital theory (MO theory or MOT) is a method for describing the electronic structure of molecules using quantum mechanics. It was proposed early in the 20th century. The MOT explains the paramagnetic nature of O2, whic ...
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
Isoelectronicity is a phenomenon observed when two or more molecules have the same structure (positions and connectivities among atoms) and the same electronic configurations, but differ by what specific elements are at certain locations in th ...
with the carbonate ion.
Similarly, the two-fold symmetrical structure of a
carboxylate
In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, (or ). It is an anion, an ion with negative charge.
Carboxylate salts are salts that have the general formula , where M is a metal and ''n'' is 1, 2,... ...
group,, 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 can be seen as the result of removing all
proton
A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
s from a corresponding
acid
An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...
C
''x''H
''n''O
''y''. Carbonate , for example, can be seen as the anion of
carbonic acid
Carbonic acid is a chemical compound with the chemical formula . The molecule rapidly converts to water and carbon dioxide in the presence of water. However, in the absence of water, it is quite stable at room temperature. The interconversion ...
H
2CO
3. Sometimes the "acid" is actually an
alcohol
Alcohol may refer to:
Common uses
* Alcohol (chemistry), a class of compounds
* Ethanol, one of several alcohols, commonly known as alcohol in everyday life
** Alcohol (drug), intoxicant found in alcoholic beverages
** Alcoholic beverage, an alco ...
or other species; this is the case, for example, of
acetylenediolate that would yield
acetylenediol C
2H
2O
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 can be matched in principle to the electrically neutral (or
oxidized
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
) variant C
''x''O
''y'', an
oxocarbon
In chemistry, an oxocarbon or oxide of carbon is a chemical compound consisting only of carbon and oxygen. The simplest and most common oxocarbons are carbon monoxide (CO) and carbon dioxide (). Many other stable (practically if not thermodynamica ...
(
oxide
An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...
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 correspond to the even less stable
1,2-dioxetanedione C
2O
4; and the stable
croconate anion corresponds to the neutral
cyclopentanepentone C
5O
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 can be reduced to the
tetrahydroxybenzoquinone (THBQ) anion and then to
benzenehexolate .
[
Haiyan Chen, Michel Armand, Matthieu Courty, Meng Jiang, Clare P. Grey, Franck Dolhem, Jean-Marie Tarascon, and Philippe Poizot (2009), "Lithium Salt of Tetrahydroxybenzoquinone: Toward the Development of a Sustainable Li-Ion Battery" ''J. Am. Chem. Soc.'', 131(25), pp. 8984–8988
]
Acid anhydrides
An oxocarbon anion can also be associated with the
anhydride
An acid anhydride is a type of chemical compound derived by the removal of water molecules from an acid (chemistry), acid.
In organic chemistry, organic acid anhydrides contain the functional group . Organic acid anhydrides often form when one ...
of the corresponding acid. The latter would be another oxocarbon with formula C
''x''O
''y''−; namely, the acid minus water molecules H
2O. The standard example is the connection between carbonate and
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
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
In chemistry and biochemistry, an oligomer () is a molecule that consists of a few repeating units which could be derived, actually or conceptually, from smaller molecules, monomers.Quote: ''Oligomer molecule: A molecule of intermediate relativ ...
or
polymer
A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
. Unlike neutralization, this formal dehydration sometimes yields fairly stable oxocarbons, such as
mellitic anhydride C
12O
9 from
mellitate via
mellitic acid C
12H
6O
12
Hydrogenated anions
For each oxocarbon anion there are in principle ''n''−1 partially hydrogenated anions with formulas , 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 is commonly called
bicarbonate
In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula .
Bicarbonate serves a crucial bioche ...
, and hydrogenoxalate 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 , a singly ionized version of rhodizonate.
[
Richard B. Wyrwas and Caroline Chick Jarrold (2006), "Production of from Oligomerization of CO on Molybdenum Anions". ''J. Am. Chem. Soc.'' volume 128 issue 42, pages 13688–13689.
]
See also
*
Oxocarbon
In chemistry, an oxocarbon or oxide of carbon is a chemical compound consisting only of carbon and oxygen. The simplest and most common oxocarbons are carbon monoxide (CO) and carbon dioxide (). Many other stable (practically if not thermodynamica ...
*
Silicate
A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...
*
Sodium percarbonate
Sodium percarbonate or sodium carbonate peroxide is an inorganic compound with the formula . It is an adduct of sodium carbonate ("soda ash" or "washing soda") and hydrogen peroxide (that is, a perhydrate). It is a colorless, crystalline, hygros ...
(actually a carbonate
perhydrate)
References
{{reflist
Carbon oxyanions
Oxocarbons