# 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.
• This page tells me that transition metal lithides have been observed, although my level of knowledge isnt sufficient to explain what they're saying: pubs.acs.org/doi/abs/10.1021/jp952638g?journalCode=jpchax& Jun 26, 2017 at 5:56
• The context of that article is vastly different (computational study on discrete MLi molecules, which aren't $\ce{M+Li-}$), so it's not relevant to this question. Aug 11, 2017 at 14:02
• Perhaps some of these will prove of limited interest: doi: 10.1021/ja056314+; doi: 10.1002/cphc.201200805; doi: 10.1021/jp8040023; doi: 10.1146/annurev.ms.23.080193.001255; doi: 10.1063/1.2393225. Dec 6, 2017 at 20:26
• The more interesting question is how sodides can form. Sodium electron affinity is ~52 kJ/mol, but heat of vaporisation of sodium metal is 72 kJ/mol. May 12, 2019 at 11:28

• 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) Oct 28, 2017 at 16:35