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

Abstract

It has been suggested recently that the alanes Al n H n + 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 B n H n + 2 , well known as the deprotonated B n H n 2−. To do so, the neutral Al n H n + 2 have been considered as Al n H n 2− + 2H +. The additional hydrogens donate their electrons to the Al n H n 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 Ga n H n + 2 are studied at high levels of calculational theory and we investigated the applicability of the W–M rule to the alanes and gallanes A n H n + 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.