Borane skeletal bond orders

The average edge bond order for a particular closo species can be calculated by dividing the total number of skeletal bond pairs ( 4- 1) by the number of edges, which, for exclusively triangular-faced poly-hedra with n vertices, is (3 — 6). This leads directly to the formal boron-boron bond order for those borane anions (ITU) and... 

The third row of bond networks in Figure 3.7, which are those possible for a tervalent atom with five neighbors, leads to a mean bond order of 13/30, that is, 0.433 , appropriate for all of the skeletal atoms in an icosahedron as in the borane dianion [B H JA For the carboranes 1,2-, 1,6-, and l,12-C2BioHi2, however, bonds to the carbon atoms can be expected to have orders greater than this, and bonds between boron atoms will be of lower order, as the more electronegative carbon atoms draw more of the skeletal electrons toward themselves, so increasing the orders of the bonds they form. 

Figure 3.8. B-C and B-B bond orders calculated and CH and BH group charges for closo boranes and closo carboranes [B H ]2 [1-CB iHJ" and 1. //-(CB LI, show the skeletal electron distributions in these systems.

Exo-polyhedral interactions (Figure 5.19) do not affect the skeletal bonding in individual monomeric units and hence Wade s rales can stiU be applied. These macro-polyhedral boranes where Wade s rule is apphcable can be called connected polyhedral boranes, in contrast to condensed polyhedral boranes in which the monomeric units share one or more boron vertices and do not obey Wade s rale. In order to be applicable to condensed polyhedral boranes as well, Wade s rule can be extended by incorporating a new variable, m, corresponding to the number of polyhedra. In Wade s n-i-l-i-p rale, 1 is added to include the core BMO. If m closo clusters condense to form a condensed polyhedral borane, there will be m core BMOs. Thus, the n-El-Ep rale can be modified to where m is the... 

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