Metal complexes of borohydride
Metal complexes of borohydride refers to coordination complexes containing the borohydride ligand. The inventory is in the hundreds. Although these compounds have few practical applications, they have attracted much attention for their unusual structures.
Bonding modes
The tetrahedral anion BH4- is isoelectronic with methane but more electron-rich owing to the electropositive character of boron and the negative charge. It binds to soft metal centers. Borohydride binds metals by forming M-H-B linkages. A variety of bonding modes are observed: κ1-, κ2-, and κ3- in which the BH4- is bonded via one, two, and three H atoms, respectively. Examples include Cu3, Cu2, and the homoleptic complexes M4. The latter highlight the ability of borohydride, which is compact, to give complexes of very high coordination numbers, Borohydride often functions as a bridging ligand.Stereodynamics
Borohydride ligands characteristically exhibit fluxionality. They are subject to rapid "bridge-terminal exchange". For example, the room-temperature 1H NMR spectrum of - shows only one hydride signal. At low temperatures, two signals in a ratio of 1:3 are resolved.Preparation
Commonly, borohydride complexes are prepared by salt metathesis reactions using potassium borohydride or sodium borohydride:The homoleptic actinide derivatives are produced using aluminium borohydride:
The metathesis is accompanied by redox in the case of Ti:
Some metal hydride complexes react with sources of borane as well to give borohydrides.