Nido borane

Arachno Clusters 2n + 6 Systems). In comparison to the number of known closo and nido boranes and heteroboranes, there are relatively fewer arachno species. Partly because of the lack of a large number of stmctures on which to base empirical rules, arachno stmctures appear to be less predictable than their closo and nido counterparts. For example, there are two isomeric forms of one with the arachno [19465-30-6] framework shown... 

Tab. 2.1-1. Electron pair counts for conde nsed nido-boranes. ...

Fia. 2. The nido-boranes BsHg, BgHio, and B10H14, showing the fundamental polyhedra. 

Fig. 2 The structures of the known nido boranes B H 44 (n = 5, 6, 8, 10). In each structure, the four bridging hydrogen atoms are shown but not the terminal hydrogen atoms...

Chemical bonding in the skeletons of some nido-boranes (large circles represent BH group, small circles represent H atoms in BHB bonds) ... 

Similarly we can show that in case of nido boranes with n atoms we should have (2n + 4) skeletal bonding electrons and in case of arachno boranes with n atoms we must have (In + 6) skeletal bonding electrons. 

No. of Skeletal Bonding Pairs Basic Polyhedron closo- Borane nido- Borane arachno- Borane hypho- Borane... 

The predicted structures for B8H12, a nido-borane in terms of electron counting, are obtained by the removal of a 5- or 6-coordinate vertex from [B9H9]2, i.e., (a) or (b) in Fig. 26 however, the observed structure of B8H12 in the solid state is the nrac/zno-structure shown in Fig. 27128 . Some bond distances and angles are given in Tables 7 and 8. 

The structures of boranes can be grouped into several classifications. If the structure contains a complete polyhedron of boron atoms, it is referred to as a closo borane (closo comes from a Greek word meaning closed ). If the structure has one boron atom missing from a comer of the polyhedron, the structure is referred to as a nido borane (nido comes from a Latin word for nest ). In this type of structure, a polyhedron having n comers has (n - 1) comers that are occupied by boron atoms. A borane in which two comers are unoccupied is referred to as an arachno structure (arachno comes from a Greek word for web ). Other types of boranes have structures that are classified in different ways, but they are less numerous and will not be described. 

The syntheses of the heteroboranes often involves the addition of heteromolecules to nido-boranes, in so called cage expansion reactions. Two of the most used borane precursors are pentaborane(9) (nido-BsHg) and decab-orane(14) (niT/o-BioHn) both of which are shown in Figure 2. 

Note Nido boranes may also have the formulas B H +3 and 1 arachno boranes may also... 

As an alternative to the reductive generation of a nido borane or carborane cluster from a closo parent, one can regard a nido species as the product of decapitation or deboronation of a closo parent. Formal removal of a B kf unit, normally from a high connectivity site in a closo borane or carborane, followed by protonation of the anionic residue around the open face generated, leaves a neutral nido residue. 

The structural patterns illustrated in Figure 3.1, and the shapes shown in column 3 for arachno species, are essentially those first suggested by Williams in 1971,2 and have stood the test of time remarkably well. However, the shapes shown should be regarded as illustrative rather than definitive. As with the nido species, alternative shapes to those shown in Figure 3.1 are possible for arachno species, with different sites left vacant on the parent deltahedron. We have already noted that many nido boranes and carboranes with eight skeletal atoms show deviations in their atom connectivities from those shown in Figure Nevertheless, apart from these particular exceptions,... 

Such localized bond networks as those in Figure 3.10 provide useful insight into the electron distribution in nido boranes and carboranes, but do not... 

The second compound, B5H9, is an example of a compound with n = 5, so it is a nido borane. Its name is... 

A useful route to B-B bonded conjuncto-boranes involves the photolysis of parent nido-boranes. Thus, ultraviolet irradiation of B5H9 (9) yields the three isomers of conjuncto-Q o f,... 

Now let us consider similar localized structures for nido boranes, which have 2n + 4 skeletal electrons and are formally derived from the hypothetical B H tetraanions by protonation of one or more B-B single bonds. The structures of the nido boranes may also be derived from a deltahedron by removal of a single vertex. " ... 

The parent polyhedron has 12 comers, so B11H13 one less boron than 12), is a nido borane. 

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