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
    Commented May 10, 2019 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. $\endgroup$ Commented May 11, 2019 at 9:47

2 Answers 2


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 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 (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.


  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
  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
  4. Inorganic Chemistry, Gary Wulfsberg, University Science Books, 2000
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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
    Commented May 11, 2019 at 10:15
  • $\begingroup$ I cannot trace the ebook reference to a primary source because the primary references are on a different page and the book is behind a paywall. So can you please provide the primary source? $\endgroup$ Commented Apr 30, 2023 at 20:52
  • $\begingroup$ @OscarLanzi Commented in the other answer. $\endgroup$ Commented May 1, 2023 at 6:16
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    $\begingroup$ Note that IE of Cs+ is near identical to IE of He. $\endgroup$
    – Poutnik
    Commented May 1, 2023 at 9:32
  • $\begingroup$ @Poutnik the high second ionization energy of Cs is indeed unfavorable, but the general agreement is that caesium in higher oxidation state will form covalent bonds. This would mitigate the ionization energy effect. $\endgroup$ Commented Dec 13, 2023 at 19:49

Not known compounds, but Moock and Seppelt[1] 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 suggests that a francium(V) fluoride complex may he possible, but this is not experimentally verified.


  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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