Are closed clusters expected from the (n + 1) skeletal electron pairs rule in alanes and gallanes? A DFT structural study of AnHn + 2 (A = Al, Ga, and n = 4–6)

It has been suggested recently that the alanes AlnHn + 2 can be treated by the polyhedral skeletal electron pair theory (PSEPT) of Wade and Mingos (W–M) as it was successful for their borane congeners such as BnHn + 2, well known as the deprotonated BnHn2−. To do so, the neutral AlnHn + 2 have been considered as AlnHn2− + 2H+. The additional hydrogens donate their electrons to the AlnHn polyhedral framework and according to the n + 1 electron pairs rule; these clusters should have closo-polyhedral structures. In this work the homologous gallanes, the structures and stabilities of GanHn + 2 are studied at high levels of calculational theory and we investigated the applicability of the W–M rule to the alanes and gallanes AnHn + 2 (n = 4–6; A = Al, Ga). It will be shown that the presence of bridging hydrogen atoms reduces the compactness of the corresponding polyhedron and so these species do not have the closed structures. The computations were performed at B3LYP/6-311+G(d,p), BPW91/6-311G(d,p) and B3LYP/6-311+G(3df,2p) levels of theory. Our interest in these compounds includes their potential use as hydrogen storage species and future clean sources of energy.

Unlike boranes BnHn2–, alanes and gallanes with empirical formula AnHn+2 (A=Al, Ga) do not respect the Wade–Mingos rule although they have the right valence electrons number, as it was previously suggested.Figure optionsDownload as PowerPoint slideResearch highlights
► AlnHn+2: n+1 e-pairs rule predicts a closed cluster but the open one is stable.
► GanHn+2: the favored cluster is totally open; the closed one doesn’t exist on PES.
► So what about the applicability of such rule?