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
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 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
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]
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
Metallaboranes, complexes with more complex boron hydrides.
^ abMarks, T. J.; Kolb, J. R. (1977). "Borohydride". Chem. Rev. 77: 263. doi:10.1021/cr60306a004.
^ 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.
^Franz, H.; Fusstetter, H.; Nöth, H. (1976). "Äther-Addukte von Tris(boranato)-titan(III) und dimere Alkoxy-bis(boranato)-titan(III)-Verbindungen". Z. Anorg. Allg. Chem. 427: 97–113. doi:10.1002/zaac.654270202.
2.svg)
