  Crystal chemistry of M(BH4)n, where M is a 2nd-4th period element, is
reviewed. It is shown that except certain cases, the BH4 group has a nearly
ideal tetrahedral geometry. Corrections of the experimentally determined
H-positions, accounting for the displacement of the electron cloud relative to
an average nuclear position and for a libration of the BH4 group, are
considered. Recent studies of structural evolution with temperature and
pressure are reviewed. Some borohydrides involving less electropositive metals
(e.g. Mg and Zn) reveal porous structures and dense interpenetrated frameworks,
thus resembling metal-organic frameworks (MOFs). Analysis of phase transitions,
and the related changes of the coordination geometries for M atoms and BH4
groups, suggests that the directional BH4...M interaction is at the origin of
the structural complexity of borohydrides. The ways to influence their
stability by chemical modification are discussed.
