Borane structure

Simlilar conclusions are indicated for the reaction product obtained from dichloro-acetonitrile and trichloroborane. For the product obtained from chloroacetonitrile and trichloroborane, however, a nitrile-borane structure is found which, according to the authors 481, might depend critically on the reaction conditions. 

The shapes of the deltahedra in the apparently hypoelectronic clusters of the heavier group 13 metals bear an interesting relationship to their electron counts. All such hypoelectronic deltahedra can be derived formally from the standard closo-borane structures (Figure 1-2) by flattening one or more vertices towards... 

This paper subscribes to the third viewpoint, and is based on an empirical approach that involves coordination number pattern recognition (CNPR). It is a simplistic approach, yet it apparently accommodates most if not all carborane and borane structures. For compounds that are still controversial and for compounds that have not yet been discovered or characterized, the CNPR thesis frequently predicts different structures, or at least fewer candidates, than do any of the theoretical treatments. 

What is thus offered is in effect a back of the envelope systematization of carborane-borane structures of predictive and teaching utility. 

Application of the Wade-Mingos Rules to Polyhedral Boranes Nido and Arachno Borane Structures... 

The following simple rules must hold for the borane structures ... 

Boranes are recognized as clusters of boron atoms which represent triangularfaced polyhedra. It was Williams62 in 1970 who pointed out in a classic paper that the structures of all boranes are derived from those of the closed polyhedral borane anions, or carboranes, with vertices ranging from 5 to 12. These regular polyhedra are shown in Fig. 5 and represent the structures of the closo-boranes (or carboranes) from which all other borane structures are derived. 

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. 

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. ... 

The closo-, nido-, arachno-borane structural paradigm... 

Until now we considered the topology of various organic molecules. In inorganic chemistry the topological methods are not so popular. Therefore, in this chapter we shall confine ourselves to only two topics, the topological theory of borane structure and the discussion of the interrelationship between topology, symmetry, and electronic structure of coordination compounds. 

Special prefixes closo-, nido- arachno-, catena-, hypho-, dado-, isodoso-, isonido-, isoarachno-, canasto-, anello-, predoso-, hyperdoso-, pileo-, and conjuncto- are found in the literature to describe borane structures. 

Together, the closo stractures make up only a very small fraction of all known borane species. Additional structural types can be obtained by removing one or more comers from the closo framework. Removal of one comer yields a nido (nest-like) stmc-ture, removal of two comers an arachno (spiderweb-like) stracture, removal of three comers a hypho (net-Uke) stracture, and removal of four comers a klado (branched) structure. Examples of three related closo, nido, and arachno borane structures are... 

FIGURE 15-13 Closo, nido, and arachno Borane Structures. 

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