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 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:
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]
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.[2]
The suboxide of carbon adopts an unremarkable structure. As for related organic cumulenes (e.g. ketene), C3O2 obeys the octet rule.
Subnitrides are also known. For example, Na16Ba6N features a nitride-centered octahedral cluster of six barium atoms embedded in a matrix of sodium.[1]