Supraicosahedral boranes

A question of interest in deltahedral borane chemistry is whether supraicosahedral boranes B H 2 (n > 13) can be prepared and whether the Frank-Kasper polyhedra (Figure 1) are suitable models for their structures. The supraicosahedral metallacarborane (C5H5)2Co2B10C2H,2 is known21 and has a structure based on the Frank-Kasper bicapped... 

It is now well established experimentally that the range of deltahedral structures possible for metallaboranes and metallacarboranes extends beyond the icosahedron to include 13- and 14-vertex deltahedra,as will be outlined later (see Section 3.10). It is also known, from calculations and some experiments, that polyhedral structures with some nontriangular faces (and so not fully deltahedral) may be preferred for boranes and supraicosahedral carboranes.What structures are experimentally possible for carboranes themselves beyond the icosahedron has yet to be fully explored. We focus here on systems with up to 12 skeletal atoms. 

Structurally, metallacarborane chemistry is the most fully documented area of carborane chemistry. There is not enough space here for a comprehensive survey, but Tables 3.2 and 3.3 and Figure 3.22 show the formulae and skeletal structures of some representative examples. More comprehensive compilations of structural data are to be found in References 1-3 and 199. Metals have been incorporated into the whole range of known carborane deltahedra, and indeed their presence has allowed supraicosahedral systems to be made that are without precedent among metal-free carboranes or boranes, as outlined below. 

Now that access into supraicosahedral carborane chemistry has been achieved, it appears likely that more higher carboranes will be prepared, and that further ingenious synthetic routes to such systems will be found. Though the stabilities of the products may not rival those of icosahedral carboranes, it will be interesting to see whether many of the higher carboranes or boranes envisaged by Lipscomb,--- Hoffmann,Schleyer,34,35,55,22 230... 

The first detailed theoretical study on the supraicosahedral boranes (13 < n < 24) was... 

In subsequent work shortly after this original report, Lipscomb and co-workers performed partial retention of diatomic differential overlap (PRDDO) calculations on these supraicosahedral boranes. This work identified three examples of supraicosahedral boranes, namely, BieHie, BigHig, and B22Fl22, where the dianions (i.e., z = 2) were found to undergo Jahn—Teller distortions. The full molecular symmetries were obtained by considering the structure to be a neutral one with only 2n skeletal electrons. 

These supraicosahedral boranes were subsequently investigated by Fowler using the pairing principle from tensor surface harmonic theory (section II.E). The predicted anomalous electron counts for BieHie, BigHig, and B22H22, namely, 2n or 2/7 -I- 4 rather than 2n + 2 skeletal electrons, were shown to be forced by symmetry for either T or Td clusters with an odd number of sets of four equivalent cage atoms or Cm or Cmv m > 3) clusters with an odd number of cage atoms on the Cm axis. 

More recently, the still experimentally unknown supraicosahedral boranes n = 13—17) were... 

Bi2Hi2 was found to be especially unfavorable thermodynamically, which may account for the fact that no metal-free binary supraicosahedral boranes are known. The geometry optimization in these computations led to the Frank—Kasper poiyhedra (Figure 11) for BhHh and BisHis " but not for BieHie, which had an optimized structure with two square faces. This computational study suggests that the supraicosahedral boranes n = 13—17)... 

Recent calculations by McKee, Wang, and Schleyer suggest the possibility of synthesizing neutral supraicosahedral boranes of the type B Hn. In particular, a capped icosahedral structure of neutral... 

B13H13 is expected to be exceptionally stable for a supraicosahedral borane since it combines the high stability of the Bi2Hi2 icosahedron with a BH + cap. 

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