Metal complexes of borohydride
Metal complexes of borohydride refers to coordination complexes containing the borohydride (BH4-) ligand. The inventory is in the hundreds.[1] Although these compounds have few practical applications, they have attracted much attention for their unusual structures.[2][3]
Bonding modes
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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 Cu(κ1-BH4)(PMePh2)3, Cu(κ2-BH4)(PPh3)2, and the homoleptic complexes M(κ3-BH4)4 (M = Zr, Hf, Np, and Pu). 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.[1]
Stereodynamics
[edit | edit source]Borohydride ligands characteristically exhibit fluxionality. They are subject to rapid "bridge-terminal exchange". For example, the room-temperature 1H NMR spectrum of [Ti(CO)4(κ3-BH4)]- shows only one hydride signal. At low temperatures, two signals in a ratio of 1:3 are resolved.[6]
Preparation
[edit | edit source]Commonly, borohydride complexes are prepared by salt metathesis reactions using potassium borohydride or sodium borohydride:[2][3]
- CuCl(PMePh2)3 + NaBH4 → Cu(κ1-BH4)(PMePh2)3 + NaCl (Me = CH3, Ph = C6H5)
The homoleptic actinide derivatives are produced using aluminium borohydride:
- AnF4 + 2 Al(BH4)3 → An(κ3-BH4)4 + 2AIF2BH4 (An = actinide metal)
The metathesis is accompanied by redox in the case of Ti(IV):[7]
- 2 TiCl4 + 8 LiBH4 + 2 thf → 2 Ti(κ3-BH4)3(thf) + + 8 LiCl + H2 + B2H6 (thf = tetrahydrofuran)
Some metal hydride complexes react with sources of borane as well to give borohydrides.
Applications
[edit | edit source]Metal complexes of borohydride have received some attention because they are volatile. The borohydrides of the actinides were investigated for isotope separation during the Manhattan Project.[1] Some borohydride complexes have been used as hydride reducing agents.[8]
See also
[edit | edit source]- Metallaboranes, complexes with more complex boron hydrides.
References
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