Borane Anions and Metallo-derivatives

Borane Anions and Metallo-derivatives.—Greenwood and Ward have reviewed the synthesis and structures of metalloboranes, with a discussion of metal-boron bonding.29 

In a review of the mechanisms of homogeneous reductions of inorganic species by tetrahydroborates, Hanzlik was able to compare the mechanism of the redox process proper with the homo- and hetero-geneous oxidation of alkali-metal tetrahydroborates.30 

The nB and H n.m.r. spectra of THF solutions containing LiBH4 and LiBD4 contain signals due to all of the series BH4 D (n = 0—4).31 There was evidence for an isotope shift in the B spectrum—thus the resonance due to BH3D occurs ca. 0.011 p.p.m. upheld from BH4, and J( B-H) in bh3d- is ca. 0.4 Hz less than in BH4. These are the first observations of an isotope shift in nB n.m.r. spectra. 

A study of the polymorphism of LiBHj to 45 kbar and 550 °C has revealed the existence of five solid forms, and the phase diagram has been presented.32 

Phase relationships in the LiBHt-E O-PhMe system were studied at 20°C (three phases were found .LiBH4, LiBH4,0.5Et2O, and LiBH4,Et20) and at 85 °C (only LiBH4 is present).33 

Borane Anions and Metallo-derivatiyes.—The derivatives (R4N)BH4 have been prepared conveniently and in good yield (R = Me, 71% R = Et, 81.5%) from KBH4 by ion exchange on cationic resins.  

A similar general method for the preparation of tetra-alkylammonium tetrahydroborates NR4BH4 has also been proposed, where Rt = Me4, Bu, Bu Me, Bu Et, (C6Hi3)3Et, (C6Hi3)3Bu , or (Ci2H2s)3Et. This involves the treatment of NR4I in aqueous EtOH at 0 °C with AV-17 anion-exchange resin in the BH4 form.  

Guillevic, F. Maillot, H. Mongeot, J. Dazord, and J. Cueilleron, Bull. Soc. chim. France, 1976, 1099. 

Alkali-metal tetrahydroborates reduce CrjOa at temperatures in excess of 250 °C (for LiBH4) to 430 °C (for CSBH4), in the same manner as shown in equation (6).  

Marynick has reviewed the structural possibilities for Be(BH4)2, especially the question of C 3v versus D34 symmetry. The results of M.O. calculations indicated that the latter was more likely, but the predicated differences in energy were very small. It was suggested that more theoretical and experimental work is needed on. the problem. This is borne out also by a re-consideration of the gas-phase electron-diffraction data on BeBjHg . These showed that at least two different molecular species were present, in variable amounts in different samples. It is therefore not possible, at present, to obtain molecular structiu-es from such data.  

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