Structure of boron hydrides

Longuet-Higgins HC, Bell RP (1943) The structure of boron hydrides. J Chem Soe 250-255... 

The actual chemical structures of boron hydrides remained a mystery for decades. The obvious analogy of the formula of diborane(6), B2H6, to ethane and of tetraborane(lO), B4H10, to butane tempted speculation that the structures were also analogous. In fact, electron diffraction studies appeared to bear this out for B2H6, whieh was incorrectly reported to have the ethane structure. 

Lipscomb authored two books, both published by W.A. Benjamin Inc. (New York). The first (1963) was entitled Boron Hydrides. The second (1969), co-authored with G. Eaton, was on NMR Studies of Boron Hydrides and Related Compounds. He published over 650 scientific papers between 1942 and 2009. His citation for the Nobel Prize in chemistry in 1976, for his studies on the structure of boranes illuminating problems of chemical bonding , echoes that of his mentor Linus Pauling in 1954, for his research into the nature of the chemical bond and its application to the elucidation of the structure of complex substances . It is for his work on the structure of boron hydrides that Lipscomb is most widely known. 

Figure 4.2 Proton-hydride (H- -H) distances (A) versus B-H- -H angles (degrees) in the Cambridge Structure Database for 18 x-ray structures of boron nitrogen compounds. The numbers correspond to the CSD file names, which can be found in ref. 7. (Reproduced with permission from ref. 8.)...

The nomenclature of boron hydride derivatives has been somewhat confusing and many inconsistencies exist in the literature. The structures of some reported boron hydride clusters are so complicated that only a structural drawing or graph, often accompanied by explanatory text, is used to describe them. Traditional nomenclature systems often can be used to describe compounds unambiguously, but the resulting descriptions may be so long and unwieldy that they are of litde use. The IUPAC (7) and the Chemical Abstract Service (8) have made recommendations, and nomenclature methods have now been developed that can adequately handle nearly all clusters compounds however, these methods have yet to be widely adopted. For the most part, nomenclature used in the original literature is retained herein. 

The basic principles of the structure and naming of boron hydride cage compounds have been discussed in Chapter 16. See particularly Fig 16.50. A very similar figure is presented in the Red Book with a similar explanation. The names of die structure types are sutnma nzcd in Table J-8J. 

The systematic principles of boron hydride structures ahd chemistry are the principal subjects of the present review. There are several reasons why these principles became clear such a long time after the discovery of these compounds (a) most of the compounds must be handled in grease-free vacuum line systems (b) some of the boron hydrides are unstable at ordinary temperatures, explosive on contact with air, and toxic (c) the structures are based on principles, still incompletely developed, of electron-deficient compounds and (d) location of the hydrogen atoms is a crucial part of the structure determinations, unlike the situation in hydrocarbons, and had to be done for the most part in X-ray diffraction studies of single crystals grown at low temperatures. 

Katz (79, 80) used 23 as a receptor for anions, studying its interaction with hydride, fluoride, and hydroxide ions. Comparison with 24 once again indicated a chelate effect compound 23 abstracting hydride or fluoride from complexed 24. The crystal structure of the hydride sponge 23, showed the hydride ion bound between the pair of boron atoms with short strong bonds. A crystal structure of the chloride complex was also elucidated and showed the same bridged structure (81). 

The hydrides of B and Si are volatile, spontaneously flammable, and readily hydrolyzed, whereas the only binary hydride of Al is a solid, polymeric material. However, structurally, the boron hydrides are unique, having unusual stoichiometries and configurations and unusual bonding because of their electron-deficient nature. 

Both —BH2 units are bound by three-center/two-electron hydride bridge structures thus, the molecule contains four terminal and two bridge H atoms bound to boron atoms first member of boron hydride class of compounds. 

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