Structural and dynamic properties of tetrahydroborate complexes

Data on the structures of tetrahydroborate complexes from X-ray diffraction analysis, neutron and electron diffraction, and IR and NMR spectroscopy are considered. It is shown that, in addition to the diversity of stabilising ligands, complexes of Group III and IV transition metals are characterised by diverse types of coordination of terminal (η¹, η² and η³) and bridging (between neutral molecular units) tetrahydroborate groups. Compounds with BH₄ groups with different denticities can be found among these complexes. Most tetrahydroborates of these metals in the oxidation states +3, +4 comply with the principle of maximum occupancy of the coordination sphere. Tetrahydroborates of Group III and IV metals exhibit clear-cut stereochemical non-rigidity (fluxionality): the protons of the BH₄ ligand are equivalent on the NMR time scale. Group I and V–VIII transition metals mainly form η²-complexes, which are stereochemically much more rigid than complexes of Group III and IV metals. It is suggested that electronic configuration of the central atom is the crucial factor determining different structures and stereochemical behaviours of the tetrahydroborate complexes. Data on the structures and properties of post-transition and s-element tetrahydroborates are also consistent with this assumption. The bibliography includes 248 references.