The hydrogen in the complex hydrides is located in the corners of a tetraeder with boron or aluminum in the center.
The negative charge of the anion, [BH4] and [AlH4] is compensated by a cation, e.g. Li or Na. The hydride complexes of borane, the tetrahydroborates M(BH4), and of alane the tetrahydroaluminate M(AlH4) are interesting storage materials, however, they were known to be stable and decompose only at elevated temperatures and often above the melting point of the complex.