# Why are lithides not known?

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.

## 2 Answers

One barrier is the greater tendency of lithium to form partially covalent bonds than heavier alkali metals. This is a good thing for organic chemists, since this stabilizes organolithium compounds and enhances the solubility of lithium chloride as a chloride ion source (see this answer). But it's not so hot for forming lithides. Covalent bonding to more electronegative elements, meaning to practically all other elements in this case, oxidizes the intended lithide ion. In effect the cryptand in an intended lithide is converted into one of those organolithium compounds.

Linear Christmas, in the comments to the question, hints at a way to beat this tendency. Two of the references given there, https://doi.org/10.1021/ja056314+ and https://doi.org/10.1002/cphc.201200805, involve theoretical investigations of lithides built with pyrrole-based cryptands. Such cryptands resist covalent bonding to lithide (or other nucleophilic) anions because this would destroy the aromatic of the ring and require electrons to overlap into a high-lying, strongly antibonding orbital. These compounds, or counterparts with heavier alkalide ions, draw interest for their expected nonlinear optical (NLO) properties, making them synthetic targets. So, stay tuned.

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.

• 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) – Mithoron Oct 28 '17 at 16:35
• Downvoting but not delete-voting because wrong answers should stay but clearly be labelled as wrong. – Jan Oct 29 '17 at 1:50