Closo-Boranes skeletal bonding electron pairs

The problems outlined in the previous section can be avoided if, instead of allocating the skeletal bonding electron pairs to localized bonds, one simply compares their number with the number of skeletal bonding MO s (199). The closo, nido, and arachno structures of boranes and carboranes can then be seen to reflect the numbers of skeletal bond pairs that are available to hold their skeletal atoms together. 

Table 1 shows examples of closo-, nido-, arachno- and hypho-boianes containing 6 through 13 skeletal bonding electron pairs. Where neutral borane example are not available, ionic species or heteroboranes are cited. 

This section presents the structure of the boron hybrides and is arranged in accordance with the relationship defined by Wade s Rules and expressed by Williams and Rudolph. Thus for the boranes containing six or more pairs of skeletal bonding electrons, the relationship between the structures of the closo-, nido-, arachno-and hypho-species is described. In cases where the parent borane does not exist, examples from heteroboranes with the correctly predicted structure based on Williams coordination number pattern recognition theory (CNPR) of borane structures will be described 70. Treated separately will be mono- and diborane species and also species with more than 12 boron atoms. Although there have been several reviews on the structures of the boranes in recent years none have used the current approach89. ... 

Finally, several authors including Lipscomb himself, have pointed out the relationship of the styx rules (see above) to Wade s rules55,87,88. The sum of the digits in the topological styx formula for a borane structure gives the number of skeletal electron pairs in the molecule since both approaches factor out the bonds to exopolyhedral substituents. Thus, in the general formula BpHp+q used in the styx rules, q values of 2, 4, 6 and 8 are identified with closo-, nido-, arachno-and hypho-borane structures, respectively. 

A second type of borane clusters are those with the general formula B H +4. These are classified as nido bo-ranes. Isoelectronic carboranes of the general formula C cB H +4 are produced by replacing a BH unit with a C. Nido refers to the nest-like structure of these compounds. Such a structure is obtained by the removal of one vertex from a closo structure (Fig. 22). Compounds of the general formula, B H +4 will have n electron pairs (one from each BH unit) plus four electrons or two pairs from the four additional hydrogen atoms. The total of n- -2 electron pairs will be used for skeletal bonding. Generally, any cluster compound with n -I- 2 skeletal electron pairs will adopt a nido structure. 

Cluster Shapes and Ion Packing The structures of tetrahedral trigonal bipyramidal [Es] , and bicapped quadratic-antiprismatic [Eio] clusters strictly follow Wade s rules and form 12 = 4x2 + 4) skeletal electron (ske) nido-, 12 = 5x2 + 2) ske closo-, and 22 = 10 x 2 + 2) ske doso-dusters, respectively [34]. Each Group 14 atom has four valence electrons, two of which remain as a lone pair at each vertex equivalent to the covalent exo-BH bonds in boranes and are not involved in the framework bonding. 

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