Polyboranes

Fig. 13. The stmctures of closo metallaboranes where O represents BH , CH (a) [< /(9j 0-( q -C H )Ni(B22H22)] (b) closo-l]l-[v[-Q ]) -l]l-53i] pri Closo metallaboranes can also be formed by the direct interaction of polyborane and metal carbonyl clusters. For example.

The higher polyboranes, which include many compounds with structures based on B clusters with B—B bonds bridged by hydrogen, are treated in 1.7. 

While alkynes, nitriles, and isocyanides have long been known to insert a cage carbon into polyboranes (COMC (1982) 5.4.2.6.1 and 5.4.2.6.2), the use of aldehydes to insert a cage carbon was first reported by Bernd Brellochs in 1997 (Figure 5).86 Good yields of 10-vertex monocarbaboranes were obtained from B10H14 with KOH and many aldehydes and this reaction was thus coined the Brellochs reaction. 87... 

This section reviews the developments in the chemistry of monoborane complexes of the transition metals especially borohydride and hydridoborate complexes. Although such complexes are not strictly metallaboranes in the sense that they are not cluster species, they are included here as they share many similarities with polyborane species of the transition metals such as three-center two-electron bonding. Additionally, as will be shown in Section 3.04.3.1 borohydride species can also be intermediates in the formation of larger M By clusters. In this chapter, three-coordinate monoborane species, which are best considered as cr-complexes between a transition metal and HBR2 or metal-boryl (M-B) species, are not considered. 

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 ... 

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. 

As is to be expected, all valence electrons and valence orbitals of the boron (4 orbitals) and hydrogen atoms (1 orbital) in the polyboranes and their derivatives are involved in bonding. (H atoms with 1 electron, H- ions with two electrons, B atoms with 3 electrons, NR3 molecules with two electrons, etc.). Then the following equations have to be satisfied ... 

Fig. 2.1-5. Formal description of the bonding in polyboranes, and various representations used in depicting formulae.

Another very useful rule for classifying the structures of polyboranes and hetero-boranes as well as many metal boron cluster compounds and their derivatives has been developed by Rudolph, Williams, Mingos and Wade (see Chapter 1.1.2) [4]. Today these are generally termed the Wade rules. They can be derived from the structures and electronic requirements of closed polyhedral boranes, such as an octahedron or an icosahedron, which are present in the anions B6H62 and B,2 H, 22. Since there are only exopolyhedral B-H bonds the number of electron... 

In addition to these polyhedral boranes there exist polyboranes where two (or more) polyboranes are connected by single B-B bonds or by sharing three or four boron atoms of two (or more) borane units by 3c2e bonds. In the case of B10H16 there are two B5H8 units where the apical boron atoms of B5H9 are joined by a B-B bond (l,l -isomer) or by a B-B bond between the apical boron atom of one pentaborane unit with a basal boron atom of a second pentaborane fragment... 

Fig. 2.1-7. Localized bonding description of some selected polyboranes. 

Geometrically it is impossible for polyhedral boranes where two closo-polyboranes share two common boron atoms, i.e., a common edge, to exist, because the hydrogen atoms of the neighboring BH groups would interfere sterically. For instance, if two B12H12 units are joined by a common edge this would result in H-H distances of less than 1.5 A, well below the van der Waals contacts. However,... 

In principle, several general methods are available for the synthesis of polyboranes and their derivatives, and only three will be discussed in more detail ... 

In general the deprotonation of a polyborane B H +m (to = 4,6) leads to the anions [BnHn+m-i] or [B H +m 2]2 by removal of one or two protons from a BHB 3c2e bridge with formation of a B-B single bond. Cluster expansion with a BH3 unit, usually offered as diborane in diethyl ether or tetrahydrofuran, produces borane anions [B +1H +m+2] or [B +1H +m+1]2 and these in turn on protonation give the polyboranes Bn+iHn+m+-. These may be stable species. However, in most cases they loose H2 which results in a cluster expansion by one BH unit. Several examples of this method are described in the following sections. 

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