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Suboxides are a class of
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 ...
s wherein the electropositive element is in excess relative to the “normal”
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 ...
s. When the electropositive element is a metal, the compounds are sometimes referred to as “metal-rich”. Thus the normal oxide of
caesium Caesium (IUPAC spelling; also spelled cesium in American English) is a chemical element; it has Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only f ...
is Cs2O, which is described as a Cs+ salt of O2−. A suboxide of caesium is Cs11O3, where the charge on Cs is clearly less than 1+, but the oxide is still described as O2−. Suboxides typically feature extensive bonding between the electropositive element, often leading to clusters. Examples of suboxides other than alkali metal derivatives: * Carbon suboxide, C3O2; * Boron suboxide, B6O; * Phosphorus suboxide, PO; * Titanium suboxides, TiO, Ti2O3, Ti3O5, Ti4O7, and Ti5O9.


Metal-containing suboxides

Suboxides are intermediates along the pathway that forms the normal oxide. Suboxides are sometimes visible when certain metals are exposed to small amounts of O2: :22 Cs + 3 O2 → 2 Cs11O3 :4 Cs11O3 + 5 O2 → 22 Cs2O Several suboxides of
caesium Caesium (IUPAC spelling; also spelled cesium in American English) is a chemical element; it has Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only f ...
and rubidium have been characterized by
X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
. As of 1997, the inventory includes the following Rb9O2, Rb6O, Cs11O3, Cs4O, Cs7O, Cs11O3Rb, Cs11O3Rb2, and Cs11O3Rb3. Suboxides are generally colored compounds indicating a degree of electron delocalisation. Cs7O has a unit cell containing a Cs11O3 cluster and 10 Cs atoms. The cluster can be visualised as being composed of three face-sharing octahedra. In the picture below the caesium atoms are purple and the oxygen atoms are red. The Cs-Cs distance in the cluster is 376 pm, which is less than the Cs-Cs distance in the metal of 576 pm. Rb9O2 and Rb6O both contain the Rb9O2 cluster, which can be visualised as two face-sharing octahedra. Rb6O can be formulated as (Rb9O2)Rb3. The Rb-Rb distance in the cluster is 352 pm which is shorter than the Rb-Rb in the metal of 485 pm. It is suggested that caesium suboxides play a role in the Ag-O-Cs (S1) and multialkali Na-K-Sb-Cs photocathodes.King, R Bruce, ed. (1994) ''Oxides: solid state chemistry'', Vol. 6 of ''WH McCarrroll Encyclopedia of Inorganic chemistry''. John Wiley and Sons.


Carbon suboxide

The suboxide of carbon adopts an unremarkable structure. As for related organic
cumulene A cumulene is a compound having three or more ''cumulative'' (consecutive) double bonds. They are analogous to allenes, only having a more extensive chain. The simplest molecule in this class is butatriene (), which is also called simply ''cumu ...
s (e.g. ketene), C3O2 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 ...
.


Related compounds

Subnitrides are also known. For example, Na16Ba6N features a nitride-centered octahedral cluster of six barium atoms embedded in a matrix of sodium.


References

{{Oxides