An oxocarbon or oxide of carbon is an inorganic compound consisting only of carbon and oxygen.^[1][2]

The simplest and most common oxocarbons are carbon monoxide (CO) and carbon dioxide (CO₂). Many other stable or metastable oxides of carbon are known, but they are rarely encountered, such as carbon suboxide (C₃O₂ or O=C=C=C=O), discovered by Brodie in 1873,^[3] and mellitic anhydride (C₁₂O₉), characterized by Meyer and Steiner in 1913.^[4][5][6] Brodie also discovered the so-called graphite oxide in 1859, but its molecular structure remained unknown until a few years ago.^[7]

	CO Carbon monoxide		CO2 Carbon dioxide		C3O2 Carbon suboxide		C12O9 Mellitic anhydride

For many decades, these few compounds were generally thought to be the only stable oxides of carbon. However, several new oxocarbons were synthesized starting in the 1960s. Some of these new oxides are stable at room temperature. Some are metastable or stable only at very low temperatures, but decompose to simpler oxocarbons when warmed. Many are inherently unstable and can be observed only momentarily as intermediates in chemical reactions or are so reactive that they can exist only in the gas phase or under matrix isolation conditions. Notable examples of the latter are dicarbon monoxide radical (:C=C=O), carbon trioxide (CO₃),^[8], carbon tetroxide (CO₄),^[9][10] and 1,2-dioxetanedione (C₂O₄).^[11][12] Some of these reactive carbon oxides were detected within molecular clouds in the interstellar medium by rotational spectroscopy.^[13]

Many hypothetical oxocarbons have been studied by theoretical methods but have yet to be detected. Examples include oxalic anhydride (C₂O₃ or O=(C₂O)=O), ethylene dione (C₂O₂ or O=C=C=O) ^[14] and other linear or cyclic polymers of carbon monoxide (-CO-)_n (polyketones),^[15] and linear or cyclic polymers of carbon dioxide (-CO₂-)_n, such as the dimer 1,3-dioxetanedione (C₂O₄)^[16] and the trimer 1,3,5-trioxanetrione (C₃O₆).^[16][17]

	C2O3 Oxalic anhydride		C2O4 1,2-Dioxetane- dione		C2O4 1,3-Dioxetane- dione		C3O6 1,3,5-Trioxane- trione		C2O2 Ethylene dione

The inventory of oxocarbons appears to be steadily growing. The existence of graphite oxide and of other stable polymeric carbon oxides with unbounded molecular structures^[7][18] suggests that many more remain to be discovered.

Contents 1 Linear carbon dioxides 2 Linear carbon monoxides 3 Radialene-type cyclic polyketones 4 New oxides 5 Polymeric carbon oxides 6 References

[edit] Linear carbon dioxides

One family of carbon oxides has the general formula C_nO₂, or O=(C=)_nO — namely, a linear chain of carbon atoms, capped by oxygen atoms at both ends. The first members are

• CO₂ or O=C=O, the well-known carbon dioxide.
• C₂O₂ or O=C=C=O, the extremely unstable ethylene dione.^[14]
• C₃O₂ or O=C=C=C=O, the metastable carbon suboxide or tricarbon dioxide.
• C₄O₂ or O=C=C=C=C=O, tetracarbon dioxide or 1,2,3-Butatriene-1,4-dione^[19]
• C₅O₂ or O=C=C=C=C=C=O, pentacarbon dioxide,^[20] stable in solution at room temp. and pure up to -90°C.^[21]

Some higher member of this family have been detected in trace amounts in low-pressure gas phase and/or cryogenic matrix experiments, specifically for n = 7^[21]:p.97 and n = 17, 19, and 21.^[22]:p.95

[edit] Linear carbon monoxides

Another family of oxocarbons are the linear carbon monoxides C_nO. The first member, ordinary carbon monoxide CO, seems to be the only one that is stable in the pure state at room temperature. Photolysis of the linear carbon doxides in a cryogenic matrix leads to loss of CO, resulting in detectable amounts of even-numbered monoxides such as C₂O, C₄O,^[23] and C₆O.^[21] The members up to n=9 have also been obtained by electrical discharge on gasous C₃O₂ diluted in argon.^[24] The first three members have been detected in interstellar space.^[24]

When n is even, the molecules are believed to be in the triplet (cumulene-like) state, with the atoms connected by double bonds and an unfilled orbital in the first carbon — as in :C=C=O, :C=C=C=C=O, and, in general, :(C=)_n=O. When n is odd, the triplet structure is believed to resonate with a singlet (acetylene-type) polar state with a negative charge on the carbon end and a positive one on the oxygen end, as in ⁻C≡C-C≡O⁺, ⁻C≡C-C≡C-C≡O⁺, and, in general, ⁻(C≡C-)_n/2C≡O⁺.^[24] Carbon monoxide itself follows this pattern: its predominant form is believed to be ⁻C≡O⁺.^[25]

[edit] Radialene-type cyclic polyketones

Another family of oxocarbons that has attracted special attention are the cyclic radialene-type oxocarbons C_nO_n or (CO)_n.^[26] They can be regarded as cyclic polymers of carbon monoxide, or n-fold ketones of n-carbon cycloalkanes. The first two members are carbon monoxide itself (CO) and the extremely unstable ethylene dione C₂O₂ or O=C=C=O.^[14]. Theoretical studies suggest that the next four members — C₃O₃, C₄O₄, C₅O₅, and C₆O₆ — may be stable, but so far they have been synthesized only in trace amounts.^[27]

	(CO)3 Cyclopropane- trione		(CO)4 Cyclobutane- tetrone		(CO)5 Cyclopentane- pentone		(CO)6 Cyclohexane hexone

On the other hand, the anions of these oxocarbons are quite stable, and some of them have been known since the 19th century.^[26] They are

• C₂O₂²⁻, acetylenediolate (Weiss and Büchner, 1963),^[28]
• C₃O₃²⁻, deltate (Eggerding and West, 1976),^[29][30]
• C₄O₄²⁻, squarate (Cohen and others, 1959),^[31]
• C₅O₅²⁻, croconate (Gmelin, 1825),^[32] and
• C₆O₆²⁻, rhodizonate (Heller, 1837).^[33][34]

The oxide C₆O₆ also forms the stable anions of tetrahydroxybenzoquinone (C₆O₆⁴⁻) and hexahydroxybenzene (C₆O₆⁶⁻),^[35] The aromaticity of these anions has been studied using theoretical methods.^[36][37]

[edit] New oxides

Many new stable or metastable oxides have been synthesized since the 1960s, such as:

• C₁₂O₁₂ or C₆(C₂O₄)₃, hexahydroxybenzene trisoxalate (1967); stable as a tetrahydrofuran solvate.^[38]
• C₁₀O₁₀ or C₆O₂(C₂O₄)₂, tetrahydroxy-1,4-benzoquinone bisoxalate (1968); stable as a tetrahydrofuran solvate.^[39]
• C₈O₈ or C₆O₂(CO₃)₂, tetrahydroxy-1,4-benzoquinone biscarbonate (1984); decomposes at about 45–53°C.^[40]
• C₉O₉ or C₆(CO₃)₃, hexahydroxybenzene triscarbonate (1984); decomposes at about 45–53°C.^[40]
• C₂₄O₆ and C₃₂O₈, respectively cyclic trimer and tetramer of the biradical 3,4-dialkynyl-3-cyclobutene1,2-dione -C≡C-(C₄O₂)-C≡C- (1990);^[41]
• C₄O₆, dioxane tetraketone or dimeric oxalic anhydride (1998); stable in Et₂O at −30°C, decomposes at 0°C.^[42]
• C₁₂O₆, hexaoxotricyclobutabenzene (2006)^[43][44]

	C10O10 Tetrahydroxy- 1,4-benzoquinone bisoxalate		C8O8 Tetrahydroxy- 1,4-benzoquinone biscarbonate		C4O6 Dioxane tetraketone
	C12O12 Hexahydroxybenzene trisoxalate		C9O9 Hexahydroxybenzene triscarbonate		C12O6 Hexaoxotricyclo- butabenzene
	C24O6 Tris(3,4-dialkynyl- 3-cyclobutene- 1,2-dione)		C32O8 Tetrakis(3,4-dialkynyl- 3-cyclobutene- 1,2-dione)

[edit] Polymeric carbon oxides

Carbon suboxide spontaneoulsy polymerizes at room temperature into a carbon-oxygen polymer, with 3:2 carbon:oxygen atomic ratio. The polymer is believed to be a linear chain of fused six-membered lactone rings, with a continuous carbon backbone of alternating single and double bonds. Physical measurements indicate that the mean number of units per molecule is about 5–6, depending on the formation temperature.^[18][45]

	Terminating and repeating units of polymeric C3O2.[18]

	Oligomers of C3O2 with 3 to 6 units.[18]

Carbon monoxide compressed to 5 GPA in a diamond anvil cell yields a somewhat similar reddish polymer with a slightly higher oxygen content, which is metastable at room conditions. It is believed that CO disproportionates in the cell to a mixture of CO₂ and C₃O₂; the latter forms a polymer similar to the one described above (but with a more irregular structure), that traps some of the CO₂ in its matrix.^[46][47].

Another carbon-oxygen polymer, with C:O ratio 5:1 or higher, is the classical graphite oxide^[7] and its single-sheet version graphene oxide.

[edit] References

v • d • e Oxocarbons Common oxides CO2 · CO Exotic oxides C2O2 · C3O2 · C4O2 · C5O2 · C2O · CO3 Compounds derived from oxides Metal carbonyls · Carbonic acid · Bicarbonates · Carbonates

v • d • e Inorganic compounds of carbon Oxides and related Oxides (CO2, CO) · Metal carbonyls · Carbonic acid · Bicarbonates · Carbonates Other ionic compounds Cyanides · Isocyanides · Cyanates · Thiocyanates · Isothiocyanates · Carbides