Larger Boranes and Carboranes

The larger nido and arachno cages are usually obtained via degradation (in one or more steps) of icosahedral carboranes, as previously mentioned, but the smaller members are typically prepared in reactions of alkynes with neutral boranes, as in the synthesis of 2,3-RR C2B4H8 (Fig. 5-14). However, the nido-2,4-QBJHf carbons apart ion can be obtained by removal of an apex boron from closo-2,4-QB5B7 (Fig. 5-10d).8... 

In comparison with the enormous number of carboranes the family of the heavier group 14 heteroborane clusters is yet very small. Most of the heteropoly-borane skeletons with Si, Ge, Sn or Pb atoms belong to the group of larger sized closo- and nido-clusters and the chemistry of these heteroboranes is essentially concentrated on the heteroatoms. 

In Chapter 15 we observed that the 18-eleciron rule was adequate for predicting stabilities of small organometallic clusters. In this chapter we have seen that Wade s rules allow us to make predictions about borune structures based on the number of framework electrons. These rules al.so are adequate for most carboranes, metallacarboranes, and other heteroboranes.Furthermore, organometallic clusters that are not derived from boranes can be dealt with in a similar fashion. More sophisticated extensions are required for complex larger clusters. 9... 

A detailed comparison demonstrated that 12-vertex and 10-vertex c/o o-boranes, represented by carboranes D and E respectively, are larger than organic carbocycles, such as bicyclo[2.2.2]octane (C), and have higher order rotational axes °- (Figure 14.4). Calculations supported by experimental data show that the van der Waals diameter curved by the p-carboranes D and E rotating around the C C axis is 7.4 A and 7.2 A respectively, which compares to 6.7 A for bicyclo[2.2.2]octane (C) and benzene (A). ... 

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