Oxocarbon anion

Oxocarbon anion
2D diagram of mellitate C12O6−
, 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 CxOyn for some integers x, y, and n.

The most common oxocarbon anions are carbonate, CO32−, and oxalate, C2O42−. 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 CO4, that have a fleeting existence during some chemical reactions; and many hypothetical species, like CO44−, 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

Carbonate-ion-resonance-2D.pngDelocalisation and partial charges on the carbonate ion Space-filling model of the carbonate ion

so that each oxygen has the same negative charge equivalent to 2/3 of one electron, and each C–O bond hs the same average valence of 4/3. This model accounts for the observed threefold symmetry of the anion.

Similarly, in a deprotonated carboxyl group –CO2, 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 C5O2−
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 CxOyn can be seen as the result of removing all protons from a corresponding acid CxHnOy. Carbonate CO32−, 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 C2O22− 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

Every oxocarbon anion CxOyn 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;[2] oxalate C2O42− correspond to the even less stable 1,2-dioxetanedione C2O4;[3] and the stable croconate anion C5O52− corresponds to the neutral cyclopentanepentone C5O5, which has been detected only in trace amounts.[4]

Reduced variants

Conversely, some oxocarbon anions can be reduced to yield other anions with the same structural formula but greater negative charge. Thus rhodizonate C6O62− can be reduced to the tetrahydroxybenzoquinone (THBQ) anion C6O64− and then to benzenehexolate C6O66−..[5]

Acid anhydrides

An oxocarbon anion CxOyn can also be associated with the anhydride of the corresponding acid. The latter would be another oxocarbon with formula CxOyn/2; namely, the acid minus n/2 water molecules H2O. The standard example is the connection between carbonate CO32− 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 C12O126− via mellitic acid C12H6O126−[6][7][8]

Hydrogenated anions

For each oxocarbon anion CxOyn there are in principle n−1 partially hydrogenated anions with formulas HkCxOy(nk)−, 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
is commonly called bicarbonate, and hydrogenoxalate HC2O
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 C22− and methanide C4−, could be seen as extreme cases of oxocarbon anions CxOyn, with y equal to zero. The same could be said of oxygen-only anions such as oxide O2−, peroxide, O22−, and ozonide O3.

List of oxocarbon anions

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

Diagram Formula Name Acid Anhydride Neutralized
Chemfm carbonate 2neg.svg CO2−
carbonate CH2O3 CO2 CO3
Chemfm peroxocarbonate 2neg.svg CO2−
peroxocarbonate CO3 CO4
28px CO4−
orthocarbonate C(OH)4 orthocarbonic acid CO2 CO4
Chemfm acetylene dioxide 2neg.svg C2O2−
acetylenediolate C2H2O2 acetylenediol C2O2
Chemfm oxalate 2neg.svg C2O2−
oxalate C2H2O4 C2O3, C4O6 C2O4
Chemfm dicarbonate 2neg.svg C2O2−
dicarbonate C2O2−
Chemfm cyclopropanetrione 2neg.svg C3O2−
deltate C3O(OH)2 C3O2−
mesoxalate C3H2O5
Chemfm acetylenedicarboxylate 2neg.svg C4O2−
acetylenedicarboxylate C4H2O4
Chemfm cyclobutanetetrone 2neg.svg C4O2−
squarate C4O2(OH)2 C4O2−
dioxosuccinate C4H2O6
Chemfm cyclopentanepentone 2neg.svg C5O2−
croconate C5O3(OH)2 (CO)5
Chemfm methanetetracarboxylate 4neg.svg C5O4−
methanetetracarboxylate C5H4O8
Chemfm cyclohexanehexone 2neg.svg C6O2−
rhodizonate (CO)4(COH)2 (CO)6
Chemfm cyclohexanehexone 4neg.svg C6O4−
benzoquinonetetraolate; THBQ anion (CO)2(COH)4 THBQ (CO)6
Chemfm cyclohexanehexone 6neg.svg C6O6−
benzenehexolate C6(OH)6 benzenehexol (CO)6
Chemfm ethylenetetracarboxylate 4neg.svg C6O4−
ethylenetetracarboxylate C6H4O8 C6O6
Chemfm furantetracarboxylate 4neg.svg C8O4−
furantetracarboxylate C8H4O9
Chemfm benzoquinonetetracarboxylate 4neg.svg C10O4−
benzoquinonetetracarboxylate C10H4O10 C10O8
Chemfm mellitate 6neg.svg C12O6−
mellitate C6(COOH)6 C6(COC)3

Several other oxocarbon anions have been detected in trace amounts, such as C6O
, a singly ionized version of rhodizonate.[9]

See also


  1. ^ Infrared and mass spectral studies of proton irradiated H₂O + CO₂ ice: evidence for carbonic acid, by Moore, M. H.; Khanna, R. K.
  2. ^ DeMore W. B., Jacobsen C. W. (1969). "Formation of carbon trioxide in the photolysis of ozone in liquid carbon dioxide". Journal of Physical Chemistry 73 (9): 2935–2938. doi:10.1021/j100843a026. 
  3. ^ Herman F. Cordes, Herbert P. Richter, Carl A. Heller (1969). "Mass spectrometric evidence for the existence of 1,2-dioxetanedione (carbon dioxide dimer). Chemiluminescent intermediate". J. Am. Chem. Soc. 91 (25): 7209. doi:10.1021/ja01053a065. 
  4. ^ Detlef Schröder,; Helmut Schwarz, Suresh Dua, Stephen J. Blanksby and John H. Bowie (May 1999). "Mass spectrometric studies of the oxocarbons CnOn (n = 3–6)". International Journal of Mass Spectrometry 188 (1–2): 17–25. doi:10.1016/S1387-3806(98)14208-2. 
  5. ^ 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 doi:10.1021/ja9024897
  6. ^ J. Liebig, F. Wöhler (1830), Ueber die Zusammensetzung der Honigsteinsäure Poggendorfs Annalen der Physik und Chemie, vol. 94, Issue 2, pp.161–164. Online version accessed on 2009-07-08.
  7. ^ Meyer H, Steiner K (1913.). "Über ein neues Kohlenoxyd C12O9 (A new carbon oxide C12O9)". Berichte der Deutschen Chemischen Gesellschaft 46: 813–815. doi:10.1002/cber.191304601105. 
  8. ^ Hans Meyer, Karl Steiner (1913.). "Über ein neues Kohlenoxyd C12O9". Berichte der Deutschen Chemischen Gesellschaft 46: 813–815. doi:10.1002/cber.191304601105. 
  9. ^ Richard B. Wyrwas and Caroline Chick Jarrold (2006), Production of C6O6- from Oligomerization of CO on Molybdenum Anions. J. Am. Chem. Soc. volume 128 issue 42, pages 13688–13689. doi:10.1021/ja0643927

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