This thesis deals with structural topologies of different dimensionalities in novel complex hydrides based on the tetrahydroborate anion. While classical applications of hydrides such as mobile hydrogen storage are discussed, the use of hydrogen-storage incompatible heavy metals, in especial lanthanides, yields new structural features and functionalities in borohydride chemistry. In this context, the photophysical properties as well as extensive structural dynamics provide means of venturing into new fields such as solid state phosphors and solid state electrolytes. Extensive characterizations are presented to correlate physical behaviour with the crystal structure. These include synchrotron X-ray diffraction, quasi-elastic neutron scattering and vibrational spectroscopies as well as optical spectroscopy and thermal analyses. In particular, di-hydrogen contacts are revealed to play a dominant role in phase transition mechanisms and it is shown that the tetrahydroborate anion can be utilized to engineer both lattice instabilities and physical properties in perovskite type complex hydrides.