Light metal borohydrides: crystal structures and beyond

Experimental structures of M(BH4)(n), where M is a 2(nd)-4(th) period element, are reviewed with a particluar emphasize on crystal chemistry. It is shown that except certain cases, the BH4 group has a nearly ideal tetrahedral geometry. Correction of the experimentally determined H-positions allows to compare directly the results obtained by different diffraction techniques and by theoretical calculations. Analysis of coordination geometries for M and BH4, and of mechanisms of phase transitions in LiBH4, suggests that the directional BH4...M interaction is at the origin of structural complexity of borohydrides. The ways to influence their stability by chemical modification are discussed. Study of structural evolution with temperature and pressure is shown to be the way to access fundamental information on structural stability of these systems.