Aluminium - a notoriously anti-social element - might form an unexpected family of hydride clusters.

Chemists have discovered that aluminium - a notoriously anti-social element compared with its sister boron - may in fact form an unexpected family of hydride clusters.

Aluminium and boron lie in the same group of the periodic table, and have a similar electronic structure. Yet while boron and hydrogen atoms happily combine to form a series of multi-atom clusters - the boranes - aluminium seems less inclined to mingle. Simple aluminium hydrides like AlH3 and Al2H6 do exist, but that’s no equivalent to the boranes’ giddy variety. 

Xiang Li, from Johns Hopkins University, Maryland, US, and colleagues have looked into the problem. They sent pulsed plasmas of hydrogen atoms through aluminium vapourised from a metal electrode, and found that hydrogen atoms did attach to aluminium anion clusters. About 200 previously unobserved aluminium hydride anions showed up after mass spectrometry analysis. 

Concentrating on one abundant member of this new family, Al4H6-, the researchers found that the neutral cluster Al4H6 should in fact be quite stable. Its structure - with four terminal Al-H bonds and two bridging hydrogen atoms - was really quite analogous to the borane family; and could be explained by the same shorthand rules that summarise borane bonding. 

If aluminium hydride clusters like these can ever be synthesised in bulk, Li’s team speculate, they could be useful materials. From calculations, it appears that burning Al4H would release enough energy to make the cluster a possible propulsion fuel. In practice, that suggestion might never bear fruit; but the newly discovered social life of aluminium may continue to surprise us.

Richard Van Noorden