What are compounds where oxygen has oxidation state -1/3 called?

Question

I have heard of oxides, peroxides and superoxides where oxygen oxidation number is $-2, -1, -\frac{1}{2}$ respectively.

But I came across a molecule with the formula $\ce{KO_3}$, oxygen has $-\frac{1}{3}$ oxidation state; what are they called?

Answer 1

These are called ozonides, specifically ionic ozonides. The name clearly comes from the ozone molecule with which the ion is isostructural. Ionic ozonides are known primarily as salts of the alkali metals, with the referenced WP article mentioning $\ce{MO3}$ for $\ce{M}=\ce{Na, K, Rb, Cs}$. The caesium compound has also been used to make tetramethylammonium ozonide in liquid ammonia; the latter has a surprising amount of stability for an organic moiety combined with an oxidizing one:

Tetramethylammonium ozonide, which can be made by a metathesis reaction with caesium ozonide in liquid ammonia, is stable up to 348 K (75 °C):

$\ce{CsO3 + [(CH3)4N][O2] → CsO2 + [(CH3)4N][O3]}$[1]

Alkaline earth metal ozonide compounds have also become known. For instance, magnesium ozonide complexes have been isolated in a low-temperature argon matrix[2].

It is misleading to consider all oxygen atoms in the ozonide ion as having oxidation state $-1/3$ because they are not all equivalent. Like the neutral ozone molecule, ozonide ion has one central oxygen atom uniquely bonded to both others. Specifically identifying an oxidation state for each oxygen atom requires making assumptions (or doing high-powered calculations) on the distribution of the odd electron. For most purposes redox reactions involving ozonide ion may be accounted by treating the $\ce{O3^-}$ ion as a single entity with a total oxidation state of $-1$ for all three atoms.

References

1. Jansen, Martin; Nuss, Hanne (August 2007). "Ionic Ozonides". Zeitschrift für anorganische und allgemeine Chemie. 633 (9): 1307–1315. https://doi.org/10.1002/zaac.200700023

2. Wang, Guanjun & Gong, Yu & Zhang, Qingqing & Zhou, Mingfei. "Formation and Characterization of Magnesium Bisozonide and Carbonyl Complexes in Solid Argon". The journal of physical chemistry. A. 114 (2010). 10803-9. https://www.researchgate.net/publication/46392397_Formation_and_Characterization_of_Magnesium_Bisozonide_and_Carbonyl_Complexes_in_Solid_Argon