Suboxide

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 O^2-. A suboxide of cesium is Cs₁₁O₃, where the charge on Cs is clearly less than 1+, but the oxide is still described as O^2-. 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, SiO_x (x < 2)

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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]

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]

References

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