Boranes localized bonds

Localized Bonds. Because boron hydrides have more valence orbitals than valence electrons, they have often been called electron-deficient molecules. This electron deficiency is partiy responsible for the great interest surrounding borane chemistry and molecular stmcture. The stmcture of even the simplest boron hydride, diborane(6) [19287-45-7] 2 6 sufficientiy challenging that it was debated for years before finally being resolved (57) in favor of the hydrogen bridged stmcture shown. 

Solving the simultaneous equations (2a) and (2b) leads to y = 3 and t = n — 2, implying the presence of three B-B bonds and n — 2 B-B-B bonds in the boron skeleton. Since a deltahedron with n vertices has In — 4 faces, the n — 2 B-B-B bonds cover exactly half of the faces. In this sense a Kekule-type structure for the deltahedral boranes B H 2- has exactly half of the faces covered by B-B-B bonds just as a Kekule structure for benzene has half of its edges covered by C=C double bonds. In 1977 Lipscomb and co-workers [29] reported a variety of such Kekule-type localized bonding structures with the lowest energies for deltahedral boranes. These structures were computed using wave functions in the differential overlap approximation. 

III. Bonding In Boranes and Carboranes A. Localized Bond Treatments... 

Although the structural pattern outlined in the foregoing can be rationalized at a simple quabtative level by using a molecular orbital approach to the skeletal bonding of boranes and carboranes (see Section III, B) it is useful to consider first what problems are encountered if one attempts to describe the bonding in terms of localized bonds. 

In the hands of Lipscomb and others 76,143, 145a, 153,189, 190), such localized bond schemes, particularly when obtained via self-consistent field (SCF) molecular orbital (MO) treatments, have proved particularly valuable for rationalizing the shapes and interatomic distances and estimating the charge distribution in many higher boranes. 

The problems outlined in the previous section can be avoided if, instead of allocating the skeletal bonding electron pairs to localized bonds, one simply compares their number with the number of skeletal bonding MO s (199). The closo, nido, and arachno structures of boranes and carboranes can then be seen to reflect the numbers of skeletal bond pairs that are available to hold their skeletal atoms together. 

That transition metal-carbonyl clusters, which contain an apparent abundance of electrons, might have much in common with boranes and carboranes, notorious for their deficiency of electrons, appears at first sight improbable. However, the structural and bonding relationship between them becomes apparent if one considers certain metal-carbonyl clusters for which localized bond treatments are unsatisfactory. 

Molecular Orbital Descriptions. In addition to the localized bond descriptions, molecular orbital (MO) descriptions of bonding in boranes and carboranes have been developed (4). Early work on boranes helped develop one of the most widely applicable approximate MO methods, the extended Htckel method. Molecular orbital descriptions are particulady useful for c/oso molecules where localized bond descriptions become cumbersome because of the large number of resonance structures that do not accurately reflect molecular symmetry. Such descriptions show that the highest occupied MO (HOMO) is degenerate for most deltahedral B H molecules, but that a closed shell is obtained for the corresponding [B H l2 anions. After... 

LOCALIZED BOND SCHEMES FOR CLOSO BORANES AND CARBORANES... 

In contrast to the limitations noted above in the value of localized bond treatments for closo carboranes, MO treatments and computations based thereon provide really useful insight into their bonding. An early illustration of their value for closo deltahedral borane dianions B 11 was provided in 1955 by... 

Such localized bond networks as those in Figure 3.10 provide useful insight into the electron distribution in nido boranes and carboranes, but do not... 

Localized Bonding Schemes for Bonding in Polyhedral Boranes 1119... 

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