Metal complexes of borohydride refers tocoordination complexes containing theborohydride (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]
Structure of M(BH4)4 (M = Zr, Hf, Np, Pu).[4] Color code: blue = M, yellow = B.FeH(BH4)(dmpe)2,[5] a complex of κ1-BH4- (Me = CH3).
The tetrahedral anion BH4- is isoelectronic withmethane but more electron-rich owing to the electropositive character ofboron and the negative charge. It binds tosoft 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 highcoordination numbers, Borohydride often functions as abridging ligand.[1]
Borohydride ligands characteristically exhibitfluxionality. They are subject to rapid "bridge-terminal exchange". For example, the room-temperature1H 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]
Metal complexes of borohydride have received some attention because they are volatile. The borohydrides of theactinides were investigated forisotope separation during theManhattan Project.[1] Some borohydride complexes have been used as hydride reducing agents.[8]
^abBesora, M.; Lledós, A. (2008). "Coordination Modes and Hydride Exchange Dynamics in Transition Metal Tetrahydroborate Complexes".Structure and Bonding.130:149–202.doi:10.1007/430_2007_076.ISBN978-3-540-78633-7.
^Bau, Robert; Yuan, Hanna S.H.; Baker, Murray V.; Field, Leslie D. (1986). "An X-Ray Study of FeH(dmpe)2(BH4): A Compound Containing a Singly-Bridged BH4 Ligand with a Bent Fe-H-B Linkage".Inorganica Chimica Acta.114 (2):L27–L28.doi:10.1016/S0020-1693(00)86434-8.
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