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When going through list of oxidation states on Wikipedia I encounterd there that sodium, potassium, rubidium and caesium exhibit oxidation states of -1, but not lithium, even though its electronegativity is more than that of the others. How's that possible? Can some one provide an example.

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You are talking about alkalides, salts where the anion is an alkali metal. There is a very brief overview on Wikipedia which also provides a couple of examples, including $\ce{[Na(\text{cryptand[2.2.2]})]+Na-}$. A good university level inorganic text book such as Greenwood and Earnshaw or Housecroft and Sharpe will provide more detail.

If you are interested in unusual oxidation states of alkali metals you might also like to know that in electrochemical experiments there is some evidence for $\ce{Cs^3+}$, which is isoelectronic with $\ce{Xe^2+}$. Again I think Greenwood and Earnshaw discusses this, but I don't have it to hand at the moment to confirm.

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  • $\begingroup$ Won't they be highly unstable and reduce everything they come in contact with. How are they stored $\endgroup$ – Harsh jain Feb 21 at 14:56
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    $\begingroup$ From memory (I'm a theoretician) you get them from solution in ammonia or amine solvents, from which some can be crystallized. I doubt many are stored long term, but now I am beginning to guess. Undoubtedly they will be good reducing agents, but they are well characterized and have been known for decades $\endgroup$ – Ian Bush Feb 21 at 15:02
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    $\begingroup$ $\ce{Cs^{3+}}$? Wow! Do you have some refs? $\endgroup$ – Oscar Lanzi Feb 22 at 1:31
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    $\begingroup$ @OscarLanzi A quick google of "cs3+ caesium" gives books.google.co.uk/… as the first hit. Unfortunately this doesn't seem to have a primary reference. Busy day, will try to search over the weekend $\endgroup$ – Ian Bush Feb 22 at 7:59
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    $\begingroup$ @ian the book gives reference 222 for the claim but I do not know how to get there without having to use the paywall. $\endgroup$ – Oscar Lanzi May 10 at 23:25

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