Polyboranes bonding

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. 

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. 

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. 

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. 

In contrast to the polyhedral boranes B H +m there exist a number of neutral boron duster molecules B X (X = Cl, Br, I, NR2, R) all of them having closed deltahedral structures in spite of the fact that the number of bonding electron pairs is only n. For this reason these homonudear cluster compounds of boron are called hypercloso polyboranes. However, there also exist anions of type B X 2 which fit Wade s rules. 

A metal atom cluster as defined by Cotton [1] is still a very broad term, because non-metal atoms can also be part of the cluster core. In this chapter mainly two types of metal atom clusters are presented the polyborane analogous polyhedral and the metalloid clusters E Rr of group 13 elements E. The structures and bonding of the polyhedral clusters with n < r are similar to those in the well-known polyboranes. 

Some molecules exist where the bonding electrons cannot be assigned to atom pairs, but belong to more than two cores, e.g. in the polyboranes. In these cases the model concept of covalently bound atom pairs as a rep-resention basis for chemical constitution using binary relations can be sustained by the assignment of fractional bond orders. 

The apparent Instability of homopolyatomic anions of elements to the left of group IV (and of polyatomic cations to the left of group V) Is thought to result from a deficiency of bonding electrons In species where the principal bonding Is considered to originate largely from p-type orbitals. The electron deficient polyboranes, with which these clusters have... 

In the early 1960s it was recognized (6,187) that there were bonding similarities between the pentagonal face of the isomeric [nirio-(y HjJ2- ions and the well-known cyclopentadienide ion (Cp ) [CydJ- (Fig. 16). The isomeric nido-C2B9H1 J2- ions, which are commonly known as dicarbollide ions, and many other carborane anions, form stable complexes with most of the metallic elements. Indeed nearly all metals can be combined with polyborane hydride clusters to produce an apparently limitless variety of cluster compounds. 

Alkyl derivatives of the nido-carborane, 2-CB5H9, are found as side products from the thermal (200°C) reaction between pentaborane (9) and acetylenes (see 5.3.2.7.3). These alkyl derivatives of 2-CB5H9 are formed from the internal 1,1 addition of two B—H bonds of the polyborane to the acetylene. 

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