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In the last few decades, many alkalides - anions of alkali metals - have been synthesised. The most famous is undoubtedly that of sodium: $\ce{[Na(\text{2.2.2-cryptand})]+Na-}$, but the alkalides $\ce{K-}$, $\ce{Rb-}$, and $\ce{Cs-}$ are all known. However, $\ce{Li-}$ is not.

James Dye writes in a 1984 review1 that for the product of the reaction

$$\ce{M(s) + N(s) + L(s) -> [ML]+N-(s)}$$

to be thermodynamically stabilised ($\ce{M}$, $\ce{N}$ are metals, and $\ce{L}$ is the macrocyclic ligand), several criteria must be met:

  1. Small lattice energies for $\ce{L(s)}$, $\ce{M(s)}$, and $\ce{N(s)}$ so that the enthalpies of sublimation will not be too large.
  2. Low ionization energy of $\ce{M}$.
  3. High electron affinity of $\ce{N}$.
  4. Large complexation energy of $\ce{M+}$ by $\ce{L}$.
  5. Large lattice energy of $\ce{[ML+]N-}$ (which depends mainly on the interionic separation).

From these given factors, I suppose the only possible explanation is the larger heat of sublimation of lithium. However, lithium is still known to form electrides in solution - compounds of the form $\ce{[ML+]e-}$. In order for these to be formed, the sublimation energy of $\ce{Li}$ still has to be overcome.

Is there a thermodynamic reason why lithides, $\ce{Li-}$, have not yet been made?

Reference

  1. Dye, J. L. Electrides, negatively charged metal ions, and related phenomena. Prog. Inorg. Chem. 1984, 32, 327–441. DOI: 10.1002/9780470166338.ch4.
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Not too complicated of an answer - don't make a mountain out of a mole-hill. This is simple nuclear stability. You can just google "Nuclear Band of Stability" and it explains the necessary balance between protons, neutrons and electrons. Li-1 would have 3 protons, 4 neutrons, and 4 electrons. Crowded around such a small nucleus, the electrons would experience too much repulsion which would cause at least one to be ejected from the cloud. Additionally, 3 measly protons does not suffice for another electron, at a greater distance, experiencing electron repulsion, to experience an even greater force by effective nuclear charge.

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    $\begingroup$ If you didn't notice, electron affinity of lithium is positive and $\ce{Li-}$ anion perfectly possible. en.wikipedia.org/wiki/Electron_affinity_(data_page) $\endgroup$ – Mithoron Oct 28 '17 at 16:35
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    $\begingroup$ Downvoting but not delete-voting because wrong answers should stay but clearly be labelled as wrong. $\endgroup$ – Jan Oct 29 '17 at 1:50

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