Arachno boranes

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

Fig. 2. Idealized deltahedra and deltahedral fragments for closo, nido and arachno boranes and heteroboranes. From left to right the vertical columns give generic closo, nido, and arachno frameworks bridge hydrogens and BH2 groups are not shown, but when appropriate they are placed around the open...

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... 

Proton Abstraction. Although the exopolyhedral hydrogens of nido and arachno boranes are generally considered hydridic, the bridge hydrogens are acidic as first demonstrated by titration of and deuterium exchange (71). Some typical reactions are... 

As with the simple boranes, the closo carboranes are generally more thermally stable than the corresponding nido and arachno species. Thermal decomposition of nido and arachno carboranes often leads to one or more closo carborane. For example, pyrolysis of 2,3-C2B4Hg is another route to 2,3-C2B3H2 [30347-95-6], l,2-C2B4Hg [20693-68-9] and l,6-C2B4Hg [20693-67-8], and 1,5-C2B3H3 [20693-66-7] (123). 

This works well for all nido- and arachno-boranes but not for the c/o o-dianions, which are much less reactive. Reactions of B2H6 with NH3 are complex and, depending on the conditions, yield aminodiborane, H2B(/r-H)(/r-NH2)BH2, or the diammoniate of diborane, [BH2(NH3)2]-[BH4] (p. 165) at higher temperatures the benzene analogue borazine, (HNBH)3, results (see p. 210). 

B5H9 also acts as a weak Brpnsted acid and, from proton competition reactions with other boranes and borane anions, it has been established that acidity increases with increasing size of the borane cluster and that arachno-boranes are more acidic than nido-horancs ... 

The structures of several other nido- and arachno- B5-B9 boranes are given on page 154 but a detailed discussion of their chemistry is beyond the scope of this treatment. Further information is in refs. 9, 11, 27, 51 and 52. 

Boranes are usually named by indicating the number of B atoms with a latin prefix and the number of H atoms by an arabic number in parentheses, e.g. B5H9, pentaborane(9) B5H11, pentaboranefl 1). Names for anions end in ate rather than ane and specify both the number of H and B atoms and the charge, e.g. BsHs" octahydropentaboratefl—). Further information can be provided by the optional inclusion of the italicized descriptors closo-, nido-, arachno-, hypho- and conjuncto-, e.g. ... 

Localized 3-centre bond formalism can readily be used to rationalize the structure and bonding in most of the non-c/r>5r>-boranes. This is illustrated for some typical nido- and arachno-horanes in the following plane-projection diagrams which use an obvious symbolism for nonnal 2-centre bonds B-B O—O, B-Hc O— , (t = terminal). 

The structural interrelationship of all the various closo-, nido- and arachno-boranes thus becomes evident a further example is shown at the foot of the page. 

Figure 3. Structure of arachno[Ptzfrf, rf-B8H toXPMezPh) ,] only the borane H atoms have been shown for clarity (4).

Fig. 1-3. Examples of nido and arachno boranes obtained by removal of vertices from the octahedron, pentagonal bipyramid, and icosahedron.

For example, both Hiickel aromatics B and C conform to the Wade-Mingos electron counting rules [2] (see Chapter 1.1.2) and to the structural systematics developed for boranes and heteroboranes [1] the hexagonal bipyramid with the apices removed is in agreement with an arachno electron count of 18 SE for (CH)g [2],... 

Scheme 3.2-13. Hydroboration of diethyl (propyn-l-yl)borane in the hydride bath via 23 and 24, followed by Et2BH-catalyzed condensation and self-assembly, leads to the 1-carba-arachno-pentaborane(lO) 25, having the Et2B substituent at the exopolyhedral carbon atom.

A classic example of the application of the above-mentioned rules is given by the borane series B6H62 (octahedron, closo), B5H9 (square pyramid, nido) and B4H10 (butterfly, that is two triangles joined by sharing an edge, arachno). The total electron counts (TEC) result in the three clusters 26, 24 and 22 whereas the skeletal electron... 

For a B Hm borane based on an anion B H 6-, an arachno-structure is preferred with B at n vertices of an (n + 2) vertices polyhedron (n + 3) pairs of framework bonding electrons are required. 

There are three main structural types, closo, nido, and arachno. Closo structures are adopted by borane anions ( = 6 -> 12),... 

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