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Suboxides are a class of oxides wherein the electropositive element is in excess relative to the “normal” oxides.[1] When the electropositive element is a metal, the compounds are sometimes referred to as “metal-rich”. Thus the normal oxide of caesium is Cs2O, which is described as a Cs+ salt of O2-. A suboxide of cesium 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 include

  • Carbon suboxide, C3O2;
  • Boron suboxide, B6O;
  • Rubidium suboxide, Rb9O2;
  • Silicon suboxide, SiOx (x < 2)


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 and rubidium have been characterized by X-ray crystallography. As of 1997, the inventory includes the following Rb9O2, Rb6O, Cs11O3, Cs4O, Cs7O, Cs11O3Rb, Cs11O3Rb2, and Cs11O3Rb3.[1]

They are generally colored compounds indicating a degree of electron delocalisation. Cs7O, has a unit cell containing a Cs10O3 cluster and 10 Cs atoms. The cluster can be visualised as being comprised of three face-sharing octahedra. In the picture below the cesium atoms are represented by the big balls and the oxygen by the small. 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. In the picture below the rubidium atoms are represented by the big balls and the oxygen by the small. Rb6O can be formulaten 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 cesium suboxides play a role in the Ag-O-Cs (S1) and multialkali Na-K-Sb-Cs photocathodes.[2]

Rb9O2 cluster Cs11O3 cluster

Carbon suboxide

The suboxide of carbon adopts an inremarkable structure. As for related organic cumulenes (e.g. ketene), C3O2 obeys the octet rule.

Related compounds

Subnitrides are also known, e.g. Na16Ba6N, which features an nitride-centered octahedral cluster of six Ba atoms embedded in a matrix of sodium.[1]


  1. ^ a b c Simon, A. ”Group 1 and 2 Suboxides and Subnitrides — Metals with Atomic Size Holes and Tunnels” Coordination Chemistry Reviews 1997, volume 163, Pages 253–270.doi:10.1016/S0010-8545(97)00013-1
  2. ^ Oxides: solid state chemistry, WH McCarrroll Encyclopedia of Inorganic chemistry. Editor R Bruce King, John Wiley and Sons. (1994) ISBN 0471 93620 0
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Suboxide". A list of authors is available in Wikipedia.
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