Homonuclear Boron Clusters

Today the chemistry of diborane and the polyboranes is well understood [2] and much of it is textbook knowledge. Therefore, after a brief survey, emphasis will focus on the development of polyhedral borane chemistry within recent decades, and even restricting discussions to homopolyboranes only certain areas can be dealt with. This incorporates synthetic procedures, the chemistry of some polyboranes and particularly polyborane anions. Other chapters of this book are devoted to heteropolyboranes such as the carbaboranes (see Chapter 3.1), azaboranes and related heteropolyboranes (see Chapter 3.3) of the main group elements. In these areas enormous progress has been achieved within the last two decades. 

In general, polyboranes can be considered to be composed of structural fragments of small boranes which by themselves may not be stable, but their existence has either been deduced from kinetic data or they have been detected as intermediates. For instance, B4H10 can be considered to be built from two BH3 and one B2H4 units, and these are indeed easily recognizable in the structure of this bor-ane. So far, four series of polyboranes are known  

B H +8 B Hm, B8H16, BioHig, B14H22, B15H23 BuHn+io BgHi8, B26H36, B40H50 

In addition there exists a boron rich polyborane such as B20H16, and this suggests that other boron rich polyboranes can be detected. Those polyboranes which are presented in square brackets have not been isolated but were either characterized 

One might expect that a large number of isomers of polyboranes should exist as the number of boron atoms increases, similar to hydrocarbon chemistry. This, however, is not the case in borane chemistry. There are indeed only a few examples of proven isomerism, for instance for the B2oHi82 anions (see Section 5.8). However, this point has been addressed in many theoretical papers, and energy differences between various isomers of a polyborane are, in general, larger than in hydrocarbons. 

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The correlation between the capacity of accumulating negative charge and the degree of homonuclear bonding in boranes agrees with the existence of the family of boranes anions [BnHn] " (n = 6-12) which are closed polyhedra with BH vertexes. In these clusters the stabilization is produced exclusively by the formation of multicenter boron-boron bonds. Borane anions have, in general, a remarkable stability. Selected examples of these boron clusters are described in Table 4.6. 

This section will focus on homonuclear neutral or anionic clusters of the elements aluminum, gallium, indium, and thallium, which have an equal number of cluster atoms and substituents. Thus, they may clearly be distinguished from the metalloid clusters described below, which in some cases have structures closely related to the allotropes of the elements and in which the number of the cluster atoms exceeds the number of substituents. The compounds described here possess only a single non-centered shell of metal atoms. With few exceptions, their structures resemble those of the well-known deltahedral boron compounds such as B4(CMe3)4 [30], B9CI9 [31] or [B H ]2 [32]. The oxidation numbers of the elements in these... 

Homonuclear clusters of the heavier elements of the third main-group aluminum, gallium, indium and thallium having direct element-element interactions form a fascinating new class of compounds. As discussed in the previous Chapter 2.3, in some cases their structures resemble those known with the lightest element of that group, boron, while in other cases novel, metal-rich compounds were obtained which do not have any analogue in boron chemistry. 

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