Can an alkalide act as a perfect Lewis base?

The Lewis basic character of alkali metals forming donor-acceptor complexes is a very rare phenomenon. No Lewis adduct with an alkalide as the Lewis basic centre has ever been reported. Herein, we theoretically designed EXH (E = Li, Na, K; X = Be, Mg, Ca) clusters which represent the first true example of Lewis adducts with alkalides as the two-electron donor basic sites. Our high level calculations reveal the formation of an unprecedented E: → XH donor-acceptor interaction. Topological analysis within the realm of the electron localization function confirms this bonding scenario. The bonding scenario is exactly replicated in all the clusters, rendering support to our proposal. The calculated bond dissociation energies are significant, suggesting their possible spectroscopic identification.