Suboxides are a class of oxides wherein the electropositive element is in excess relative to the âÂÂnormalâ oxides. When the electropositive element is a metal, the compounds are sometimes referred to as âÂÂmetal-richâÂÂ. Thus the normal oxide of caesium is Cs<sub>2</sub>O, which is described as a Cs<sup>+</sup> salt of O<sup>2âÂÂ</sup>. A suboxide of caesium is Cs<sub>11</sub>O<sub>3</sub>, where the charge on Cs is clearly less than 1+, but the oxide is still described as O<sup>2âÂÂ</sup>. 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 O<sub>2</sub>:
Several suboxides of caesium and rubidium have been characterized by X-ray crystallography. As of 1997, the inventory includes the following Rb<sub>9</sub>O<sub>2</sub>, Rb<sub>6</sub>O, Cs<sub>11</sub>O<sub>3</sub>, Cs<sub>4</sub>O, Cs<sub>7</sub>O, Cs<sub>11</sub>O<sub>3</sub>Rb, Cs<sub>11</sub>O<sub>3</sub>Rb<sub>2</sub>, and Cs<sub>11</sub>O<sub>3</sub>Rb<sub>3</sub>.
Suboxides are generally colored compounds indicating a degree of electron delocalisation. Cs<sub>7</sub>O has a unit cell containing a Cs<sub>11</sub>O<sub>3</sub> 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. Rb<sub>9</sub>O<sub>2</sub> and Rb<sub>6</sub>O both contain the Rb<sub>9</sub>O<sub>2</sub> cluster, which can be visualised as two face-sharing octahedra. Rb<sub>6</sub>O can be formulated as (Rb<sub>9</sub>O<sub>2</sub>)Rb<sub>3</sub>. 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.
The suboxide of carbon adopts an unremarkable structure. As for related organic cumulenes (e.g. ketene), C<sub>3</sub>O<sub>2</sub> obeys the octet rule.
Subnitrides are also known. For example, Na<sub>16</sub>Ba<sub>6</sub>N features a nitride-centered octahedral cluster of six barium atoms embedded in a matrix of sodium.