Valence theory boron hydride

Valence theory, boron hydride, diborane and, 124-126 electron deficiency and, 121-122 higher hydrides and, 126-128 molecular orbitals and, 128-131 three-center bond and, 122-124... 

The valence theory (4) includes both types of three-center bonds shown as well as normal two-center, B—B and B—H, bonds. For example, one resonance stmcture of pentaborane(9) is given in projection in Figure 6. An octet of electrons about each boron atom is attained only if three-center bonds are used in addition to two-center bonds. In many cases involving boron hydrides the valence stmcture can be deduced. First, the total number of orbitals and valence electrons available for bonding are determined. Next, the B—H and B—H—B bonds are accounted for. Finally, the remaining orbitals and valence electrons are used in framework bonding. Alternative placements of hydrogen atoms require different valence stmctures. 

It is also noteworthy that Alfred Stock, who is universally acclaimed as the discoverer of the boron hydrides (1912). " was also the first to propose the use of the term "ligand (in a lecture in Berlin on 27 November 1916). Both events essentially predate the formulation by G. N. Lewis of the electronic theory of valency (1916). It is therefore felicitous that, albeit some 20 years after Stock s death in 1946, two such apparently disparate aspects of his work should be connected in the emerging concept of boranes as ligands . 

The present review will be restricted to the boron hydrides themselves and their relatively simple substitution derivatives. Even the closely related, and very important, borohydrides are not reviewed and are mentioned here only insofar as they are relevant to the boron hydrides. Also, in order to restrict the review to manageable dimensions, I have omitted references to nearly all of the early structure investigations and early systematic valence theories now generally believed to be incorrect, and to claims made for the existence of new compounds when the evidence seems to be not sufficiently extensive or convincing. 

The remarkable geometrical similarities of these hydrides is very striking and will become more so as the valence theory is developed. Superposition of the ball and stick type of molecular models on one another brings out the diborane type of geometry in the open parts of these molecules, and is very suggestive of the kinds of hybridization that may be chosen for convenience about the boron atoms. 

We now outline two approaches to a description of valence in the boron hydrides. The first employs three-center bonds. This particular kind of localized molecular orbital seems most suitable for the smaller, more open hydrides. Its use in the more complex hydrides will require delocalization of the bonding electrons either by a molecular orbital modification or a resonance description. The second approach is simply that of molecular orbitals, which is particularly effective in the more condensed and symmetrical hydrides. These approaches merge as the discussion becomes more complete. It is an important result that filled orbital descriptions are obtainable for the known boron hydrides. Also some remarks about charge distribution in the boron hydrides are possible. But the incompleteness of this valence theory in this nontopological form is indicated by the lack of a large number of unknown hydrides, whose existence would be consistent with these assumptions. 

Two common features of boron hydrides (see Sections 13.5 and 13.11) are that the B atoms are usually attached to more than three atoms and that bridging H atoms are often present. Although a valence bond model has been developed by Lipscomb to deal with the problems of generating localized bonding schemes in boron hydrides, the bonding in these compounds is not readily described in terms of VB theory. The structure of B2Hg (Z>2h symmetry) is shown in Fig. 5.31. Features of particular interest are that ... 

The electronic formulation of the structure of the boron hydrides encounters a number of difficulties. The ordinary concepts of valence will not suffice to explain their stmcture this is shown by the fact that in the simplest hydride, diborane B2H6, which has 2x3 + 6= 12 electrons, as many bonds must be explained as are required for C2H6 which has two more (2x4 + 6 = 14) electrons available. Thus it is that any structural theory for these compounds requires new hypotheses. 

The theory of valence of these structures is of interest for several reasons. The hydrides themselves have an unusual set of formulas, and one might hope that a theory would correlate these and predict other members of the series. But more important, because these hydrides are electron deficient in the sense that there are more orbitals than electrons, one might hope that their electronic and geometrical structures will aid in the understanding of the large number of intermetallic compounds, and of the border line between metals and nonmetals. This interpretative problem is comparatively simpler for boron, which uses only the 2s and the three 2p orbitals, and hydrogen, which uses only the Is orbital, in the approximation discussed here. This approximation is fairly good in... 

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