Theoretical design of stable hydride clusters: isoelectronic transformation in the EnAl4−nH7+n− series

New stable hydrogen-rich metallic hydrides are designed by systematic transformations of the stable known Al4H7− species, carried out by successive isoelectronic substitutions of one aluminum atom by one E–H unit at a time (where E = Be, Mg, Ca, Sr and Ba atoms). Searches on the potential energy surfaces (PESs) of EAl3H8−, E2Al2H9−, E3AlH10− and E4H11− systems indicate that structural analogues of Al4H7− become higher energy isomers as the number of E–H units increases. The electronic descriptors: Vertical Electron Affinity (VEA), Vertical Ionization Potential (VIP) and the HOMO–LUMO gap, suggest that the systems composed of EAl3H8−, E2Al2H9−, E3AlH10−, with E = Be and Mg, would be the most stable clusters. Additionally, for a practical application, we found that the Be–H and Mg–H substitutions increase the hydrogen weight percentage (wt%) in the clusters, compared with the isoelectronic analogue Al4H7−. The good capacity of beryllium and magnesium to stabilize the extra hydrogen atoms is supported by the increment of the bridge-like E–H–Al, 3center–2electron chemical bonds. Finally, explorations on the PESs of the neutral species (using Na+ as counterion) indicate that the NaBe2Al2H9, NaBe3AlH10 and NaMg3AlH10 minimum-energy structures retain the original geometric shapes of the anionic systems. This analysis supports the potential use of these species as building blocks for cluster-assembled hydrides in the gas phase.