Are there known compounds of caesium with oxidation number >1?

Given that the left neighbor of caesium, xenon, does have fluorides and oxides, it is not inconceivable that caesium can have oxides or fluorides with an oxidation number higher than 1. Are such compounds known and synthesized? Or, alternatively, are there computations that rule out such compounds as unstable?




As noted in the answer to the [other question](https://chemistry.stackexchange.com/questions/109846/unusual-oxidation-states-of-alkali-metals/109854#109854), alkali metal can exist in higher oxidation state when bonded with polycylic multidendate ligands like cryptands etc. This is an excerpt from an eBook1:



> 
> The chemistry of group 1 elements have been dominated by +1 oxidation
> states. However, there have been indications that caesium might form
> higher oxidation species. Thus, electrochemical oxidation of
> $\ce{[CsL]PF6}$ L=18-crown-6 or cryptand-[222] gives evidence for
> $\ce{Cs^2+}$ and $\ce{Cs^3+}$. Compounds containing caesium in higher
> oxidation state is yet to be isolated.
> 
> 
> 


Caesium is predicted to form polyfluorides but is yet to be needs validated through experiments. This below is from [Wikipedia](https://en.wikipedia.org/wiki/Caesium) (also citing two references - Ref.2 and Ref.3)



> 
> caesium is predicted to behave as a p-block element and capable of forming higher fluorides with higher oxidation states (i.e., $\ce{CsF\_n}$ with n > 1) under high pressure. This prediction needs to be validated by further experiments.
> 
> 
> 


Furthermore, it has also been observed by Moock and Seepelt, *Angew, Chem. Intl. Ed. Engl.* **28**, 1676(**1989**) that $\ce{Cs+}$ can be oxidised in acetonitrile solution to $\ce{Cs^3+}$ at a potential of +3.0 V($\pu{E°}$)4.


**References**


1. *Introduction to Modern Inorganic Chemistry*, 6th edition by R.A. Mackay, W. Henderson, **2017**, CRC Press.
2. Miao, Ms. *Caesium in high oxidation states and as a p-block element. Nature* Chem 5, 846–852 (**2013**). DOI: [10.1038/nchem.1754](https://www.nature.com/articles/nchem.1754)
3. *Forcing Cesium into Higher Oxidation States Using Useful hard x-ray Induced Chemistry under High Pressure* by Sneed et. al., J. Phys.: Conf. Ser., **2017**. DOI: [10.1088/1742-6596/950/4/042055](https://iopscience.iop.org/article/10.1088/1742-6596/950/4/042055)
4. *Inorganic Chemistry*, Gary Wulfsberg, University Science Books, **2000**


 5

Not known compounds, but [Moock and Seppelt[1]](https://doi.org/10.1002/anie.198916761) have some evidence that at least some short-lived caesium(>I) species exists.


In both Ref. [1] and an earlier work cited by the authors, electrochemical techniques reveal that caesium-bearing electrolytes give an oxidation reaction thar does not appear when the caesium is replaced by potassium or rubidium. The authors note that $\ce{Cs(III) + Cs(I)}$ is more stable than $\ce{2 Cs(II)}$, so they generally speak of caesium being oxidized to the $+3$ oxidation state.


It is possible that caesium ion is actually catalyzing the oxidation of the solvent or the ligand used to complex the caesium (which would account for observations that the oxidation is irreversible in most of the experiments), but such catalysis would likely involve an intermediate caesium(>I) species whose counterpart is absent with lighter alkali metals. Given the fact that no such higher oxidation state species is as yet known as a reaction product, this hypothesis of a short-lived intermediate seems most plausible.


[Wikipedia](https://en.wikipedia.org/wiki/Francium) suggests that a francium(V) fluoride complex may he possible, but this is not experimentally verified.


**Reference**


1. Moock, K. and Seppelt, K. (1989), "Indications of Cesium in a Higher Oxidation State". *Angew. Chem. Int. Ed. Engl.*, **28**: 1676-1678. <https://doi.org/10.1002/anie.198916761>.


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