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?

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    $\begingroup$ Check out this answer and comments, to begin with. $\endgroup$
    – andselisk
    May 10 '19 at 12:32
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    $\begingroup$ @andselisk: Thanks for the link, unfortunately it is mainly about alkali anions with a short mention of ${\rm Cs}^{3+}$ that is not fleshed out. $\endgroup$ May 11 '19 at 9:47

As noted in the answer to the other question, alkali metal can exist in higher oxidation state when bonded with polycylic multidendate ligands like cryptands etc. This is an excerpt from an eBook:

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.

Also, compounds of form $\ce{CsF_n}$ is known to contain stoichiometric amount of caesium in higher oxidation state($\ce{Cs^2+,Cs^3+,Cs^4+,Cs^5+}$). Experimental calculations have confirmed the presence.(See here and here).

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($\ce{E°}$). - (See this eBook)

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    $\begingroup$ I think it is worth noting that while all these give hints that Cs+ can, under extreme conditions, be oxidised further, (to my knowledge) no compounds with Cs in a high oxidation state have actually been isolated. I also wish there were a lot more details on how those calculations were done .... $\endgroup$
    – Ian Bush
    May 11 '19 at 10:15

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