Suboxides are a class of

oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...

s wherein the

electropositive Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the ...

element is in excess relative to the “normal”

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) (or cesium in American English) is a chemical element with the 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 five elemental metals that a ...

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: *

Boron suboxide Boron suboxide (chemical formula B6O) is a solid compound with a structure built of eight icosahedra at the apexes of the rhombohedral unit cell. Each icosahedron is composed of twelve boron atoms. Two oxygen atoms are located in the interstices al ...

, BO; *

Carbon suboxide Carbon suboxide, or tricarbon dioxide, is an organic, oxygen-containing chemical compound with formula and structure . Its four cumulative double bonds make it a cumulene. It is one of the stable members of the series of linear oxocarbons ...

, C₃O₂; *

, B₆O; * Phosphorus suboxide, PO

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

and rubidium have been characterized by

X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

. 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

photocathode A photocathode is a surface engineered to convert light (photons) into electrons using the photoelectric effect. Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron be ...

s.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 In organic chemistry, 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 al ...

s (e.g.

ketene In organic chemistry, a ketene is an organic compound of the form , where R and R' are two arbitrary monovalent chemical groups (or two separate substitution sites in the same molecule). The name may also refer to the specific compound ethen ...

), 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 rul ...

Related compounds

Subnitride Subnitrides are a class of nitrides wherein the electropositive element is in excess relative to the “normal” nitrides.Simon, A. ”Group 1 and 2 Suboxides and Subnitrides — Metals with Atomic Size Holes and Tunnels” Coordination Chemistry ...

s are also known. For example, Na₁₆Ba₆N features a

nitride In chemistry, a nitride is an inorganic compound of nitrogen. The "nitride" anion, N3- ion, is very elusive but compounds of nitride are numerous, although rarely naturally occuring. Some nitrides have a find applications, such as wear-resistant ...

-centered

octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet a ...

cluster may refer to: Science and technology Astronomy * Cluster (spacecraft), constellation of four European Space Agency spacecraft * Asteroid cluster, a small asteroid family * Cluster II (spacecraft), a European Space Agency mission to study th ...

of six

barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...

atoms embedded in a matrix of sodium.

References

{{Oxides