Arachno species

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... 

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). 

Only in two cases, the eight- and nine-vertex arachno-species shown in Figure 1, have alternative geometries been found for species of the same formula type (electron count).44 75... 

Fundamental Closo species Nido species Arachno species... 

Fundamental polyhedron h Closo species (0 = 6-1) Nido species (0 = 6-2) Arachno species (0 = 6-3)... 

In related reactions, units, such as Cr(CO)3, Mn(CO)3", Fe(CO)8 , Co(C5H5) +, Mc3N, Be +, or T1+, that can supply 1 skeletal atom and three vacant AO s (no skeletal electrons) for cluster bonding can be used to fill in the vacant vertices of nido or arachno species, thereby converting them into closo or nido species, e.g. 184,194),... 

The nido ten-vertex and arachno ten-vertex deltahedral fragments are coincidently the same. Such ten-vertex nido and arachno species are not isoelectronic, although they are frequently misjudged as being isoelectronic, and a number of investigators have assigned incorrect structures based on comparison of B NMR spectra of compounds that were mistakenly thought to be isoelectronic. 

Wade also incorporates electron-rich species such as C5H5 and C4H42- as arachno species derived by removal of two nonadjacent high-coordination vertices from the pentagonal and tetragonal bipyramids, respectively, and has, moreover, correlated many aspects of seemingly unrelated classes of compounds beyond the recognized borders of carborane chemistry. 

No eleven-vertex arachno species is known, but BjoHig" (III- and IV-NIO) 129, 132) in the presence of diborane indulges in total boron and hydrogen exchange, and a transient arachno intermediate BuHie has been suggested 124). 

Thus, within the arachno species, the endohydrogens of BH2, CH2, and NH2(SH) groups are progressively more acidic the latter three are more acidic than most two-coordinate bridge hydrogens. 

The complete structural relationships among ihe closo. nido. and arachno species arc shown in Fig. 16.50. The diagonal lines connecting the species represent ihe... 

The only arachno-species known for a borane where N = 6 are [B3H8] and B3H7L. The structure of [B3H8] is that predicted for the arachno-species derived by removing the two axial vertices from [B5H5]2- and is shown in Fig. 10. 

The structural relationship between the closo-, nido- and arachno-species containing seven skeletal bonding electron pairs is shown in Fig. 13. 

The arachno-species B8H14 is known although there is little structural information available. The only structural data available are those from NMR spectra134 which suggest that species has the expected arachno-stmctuie shown in Fig. 32. 

GeC2B9Hii is equivalent to B12H14, derived from B12H12, a ctoio species. SB9H 2 is equivalent to BigHis, derived from BioHjo, an arachno species. 

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,... 

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 ),... 

Figure 3.12. The hexagonal, heptagonal, and octagonal bipyramids as the parent polyhedra from which the aromatic ring systems QH, C7H7 and Cal U are formally derived as arachno species.

Note that both descriptions of the Fe4 residue, whether as an arachno species based on an octahedron, or as a nido species based on a trigonal bipyramid, are compatible with a butterfly shape. However, the dihedral angle between the wing planes, and the M-C-M angles at the core (or equatorial) carbon atom, will differ markedly according to which description is used. A... 

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