Nido and Arachno Carboranes

In rationalizing nido structures, note that adding a pair of electrons to convert a closo dianion [B H ] into the nido tetra-anion [ causes the 

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. 

Some carboranes have formulae compatible with both classically bonded and nonclassically bonded structures and exhibit valence isomerism. The tet-racarbaborane Me4C4B6Et6 is one such. As prepared by dimerization of the small closo carborane Me2C2B3Et3 [Eq. (3.2)], it has a classical adamantane-type structure, with its four CMe units linked through BEt units (1). However, when heated it isomerizes to the expected nido carborane structure (2) with a skeletal structure like that of decaborane.  

TABLE 3.1. closo, nido, and arachno Classification of Carboranes, Carbocations, and Carbanions Containing Hypcrcarbon Atoms 

Number of Skeletal Electron Pairs Parent Deltahedron (Symmetry) Closo Species CjB,H,+2 (H ) Nido Species C,B,H,44 (H ), Arachno Species C3,H + (H ), 

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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). 

The controlled synthesis of carbons adjacent nido- and arachno-carborane anions can be achieved by the reduction of C (cage)-C (cage)-linked o-carboranes with group 1 metals.234-237 Typical examples are illustrated in Scheme 22. These have been found to undergo metallation reactions similar to their carbons apart analogs, forming full- and half-sandwich complexes of lanthanide metals.238... 

Figure 5-13 A selection of nido- and arachno-carborane structures, (a) nido- 1,2-C2B3H7. (b) nido-2-CBsH9. (c) nido-2,3-QB4H8. (d) irfo-2,3,4-C3BjH7. (e) nido-2,3,4 -C U. (f) arachno-C n. (g) nido-QB9H13. (h) arachno-C3B7H13. (i) nido-CABoH12. Compounds (h) and (i) are known only as C-substituted derivatives.

Closo, nido, and arachno carboranes are all known, most commonly containing two carbon atoms examples are shown in Figure 15-15. 

The structures of these compounds seem to dictate their methods of preparation as well as their properties. The closo carboranes are more thermally stable and less air sensitive than the corresponding nido and arachno carboranes. This is due to the fact that the nido and arachno carboranes lack... 

Figure 1 shows the known experimental geometries of carboranes. For discussion of closo-, nido-, and arachno-zlectron counts and their expected geometries, see COMC (1982) 5.4.2.2 and 5.4.2.3. Arachno-six- and seven-vertex geometries are not known experimentally for carboranes. 

The same polyhedra serve as the basis for the structures of nido and arachno compounds, too, although for these boranes and carboranes the polyhedra are incomplete. Nido structures are adopted by neutral boranes BjjHw j.4, carboranes CBy 4Hn.. 3, CsBji—... 

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. 

In 1971, a note 164) was published favoring the hypothesis that the carboranes, boranes, their isoelectronic anions, Lewis base adducts, and heteroatom-substituted analogs should be viewed as constructed about the vertices of either the most spherical series of triangular-faceted polyhedra (deltahedra) found to be characteristic of the dicarba-cZoao-carboranes (Fig. 1) or, with one lone exception, fragments of the series of deltahedra produced by the successive removal of the highest coordinated vertices that sequentially define the nido and arachno classes. This position was in conflict with the then prevalent shibboleth that all nido and arachno compounds [except B5H9 (I-N5)] had or would prove to have icosahedral fragment structures. 

The larger nido and arachno cages are usually obtained via degradation (in one or more steps) of icosahedral carboranes, as previously mentioned, but the smaller members are typically prepared in reactions of alkynes with neutral boranes, as in the synthesis of 2,3-RR C2B4H8 (Fig. 5-14). However, the nido-2,4-QBJHf carbons apart ion can be obtained by removal of an apex boron from closo-2,4-QB5B7 (Fig. 5-10d).8... 

For polyhedral clusters (sometimes called deltahedral, because the faces are all triangles resembling the Greek letter delta) the ancestor of all electron counting schemes is the correlation proposed by Wade between borane (or carborane) cages and metal carbonyl cages. Wade first drew attention to the similarity of a M(CO)3 unit and a BH (or CH) unit, a relationship that we would now call isolobality (Section 1-6). He then proposed that the 2n + 2 rule for closo boranes (Chapter 5) would also apply to closo metal cluster species such as [Os CO) ]2, and that 2n + 4 and 2n + 6 electron counts would, similarly, be appropriate for stable M clusters with nido and arachno structures. Hydrogen atoms are assumed to contribute one electron each, an interstitial carbon atom four electrons, and so on. 

Closo, nido, and arachno boranes form series with formulae B H2 +4, and B H2 +6 coirfirming their status as formally subvalent componnds. Boranes and carboranes (in which one or more boron atoms in the cluster have been replaced by a carbon atom) have been extensively researched and described, and will not be discussed further here. In an example of the application of Wade s rules to a heavier main group cluster, 805 nses 10 of the total 22 valence electrons in 5 lone pairs. This allows 12 electrons, or 2n 4- 2 for n = 5 for cluster bonding and specifies a closo structure, matching the observed Z>3h trigonal bipyramidal structure. 

Figure 3.1. Deltahedral and deltahedral fragment skeletal shapes of typical closo, nido, and arachno boranes and carboranes.

The relationship of nido and arachno boranes and carboranes to notional parent closo boranes and carboranes, by removal of BH units from selected sites, usually of high connectivity, can be illustrated simply by exploring how the MOs of a closo system B,I I, (or isoelectronic neutral carborane C2B, H ) will be affected by the theoretical experiment of removing one or two... 

The carboranes conform to the electronic rules given above for boranes and are known in closo. nido, and arachno structures. When a iplying the formulas to the carboranes, each C—H group should be regarded as donating three electrons to the framework count. Some carboianes provide interesting examples of the possiUe horizontal transformations of Fig. 16.50 mentioned above. For example - ... 

While the parent carboranes can be made from formaldehyde and decaborane, carboranes with substituents at the cage carbon can easily be synthesized from the appropriate aldehyde. For example, benzaldehyde and B10H14 give the zW(9-6-Ph-6-CB9Hn anion in 94% yield.92 The phenyl carborane formed is nido- not arachno- but subsequent carborane products can be made using similar reagents as for the parent arachno-6-CB9H14-.93... 

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