Closo molecules

Wade s rules only apply to clusters which may be regarded as derived from a cage, closo, molecule. However, many exotic clusters exist which are much more complicated. When such molecules may be regarded as resulting from the fusion of molecules which, separately, are derived from closo... 

Closo Clusters 2n + 2 Systems). The assignment of valence electrons and the factoring out of those electrons involved in exopolyhedral bonds provides 2n framework electrons for a B H molecule, two electrons short of the 2n + 2 closo count. In fact, stable neutral B H molecules are not... 

The product, Si4(SiBu 3)4, forms intensely orange crystals that are stable to heat, light, water and air, and do not melt below 350°. The Si-Si distances within the closo-Si4 cluster are 232-234 pm and the exo Si-Si distances are slightly longer, 235-237 pm (cf. Si-Si 235.17 in crystalline Si). Comparison with the closo-amon Si4 , which occurs in several metal silicides (p. 337) and is isoelectronic with the P4 molecule, is also appropriate. 

Molecules such as P4 and the polyanionic clusters such as Si4- or As2- that are discussed in Section 13.2 are representatives of electron precise closo clusters. Organic cage molecules like tetrahedrane (C4R4), prismane (C6H6), cubane (C8H8), and dodecahedrane (C20H20) also belong to this kind of cluster. 

The discovery of polyhedral boranes and polyhedral heteroboranes, which contain at least one atom other than in the cage, initiated a new era in boron chemistry.1-4 Most commonly, of the three commercially available isomeric dicarba-closo-dodecaborane carboranes(l,2-, 1,7-, and 1,12-), the 1,2-isomer 1 has been used for functionalization and connection to organic molecules. The highly delocalized three-dimensional cage bonding that characterizes these carboranes provides extensive thermal and kinetic stabilization as well as photochemical stability in the ultraviolet and visible regions. The unusual icosahedral geometry of these species provides precise directional control of all exopolyhedral bonds. 

Precise placement of metal complexing sites within the infrastructure of a cascade molecule is of importance from a variety of perspectives. In the construction of the above noted Micellane family (cf. Sect. 3.1), we reported the construction of dendrimers with four alkyne moieties at sites equidistant from each other in the interior (17, Fig. 8) [60]. These were treated with decaborane (B10H14) to afford 1,2-dicarba-closo-dodecaboranes (o-carboranes) [71]. Rendering boron clusters soluble in water is of interest because of their use in cancer treatment by Boron Neutron Cancer Therapy. First and second generation water-soluble dendrimers containing four and twelve precisely located boron cluster sites, respectively, were synthesized (e.g., 18). These water soluble dendrimers and their precursors were characterized by 13C-, and nB-NMR spectroscopy (Fig. 8). 

The molecular structure of the closo 12-vertex 1-SBnHn cluster (17) has been studied by electron diffraction methods augmented by ab initio calculations. Substantial distortions away from the regular icosahedron occurred by expansion of the pentagonal belt adjacent to sulfur.73 The UV-PES spectra of 17 has also been reported,71 and its microwave spectrum has been investigated and demonstrated that the molecule had C5v symmetry.74... 

For example, opening of the cluster skeleton from closo to nido to arachno will accompany the addition of electron pairs, whether by reduction reactions or by addition of Lewis base molecules that do not dislodge other Lewis bases. Cluster closing is expected to accompany the removal of electron pairs, whether by oxidation or by loss of a substituent together with the electron pair by which it was formally bound to the cluster ... 

Arachno Clusters (2 n + 6 Systems). In comparison to the number of known closo and nido boranes and heteroboranes, there are rdatively fewer arachno species. Pardy because of the lack of a large number of structures on which to base empirical rules, arachno structures appear to be less predictable than their closo and nido counterparts. For example, there are two isomeric forms of B9H15, one with the arachno [19465-30-6] framework shown in Figure 2 (33), the other with a framework more reminiscent of that shown for the nine-atom nido classification (34). Structures of arachno molecules involve the presence of even more extra hydrogens or other electron-donating heteroatoms than nido molecules. Typical examples are given in Table 1. 

There are a number of other important boron cage compounds as well as structurally similar metal atom cluster molecules that are not closed polyhedra. Generally, these may be regarded as derived from closed polyhedra by removal of one or two vertices. The diagram below illustrates how removal of one or two vertices generates the so-called nido and arachno relatives of a closo octahedral structure. 

Molecules containing more than 4 metal atoms are very often conglomerates of smaller stable fragments described above. There do exist, however, a number of highly symmetric (often closo-) aggregates containing 5, 6, or 9 nuclei. 

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