Boron hydrides closo

Polyhedral Boron Hydrides. Although relatively large quantities of polyhedral boron hydrides and carboranes have been produced under various government contracts, these materials are not currendy produced on a regular commercial basis. Pentaborane(9) nido- and decaborane(14) and carboranes, such as the ortho and meta-carboranes closo-1 1 well as various other derivatives, are... 

PORONCOMPOUNDS - BORON HYDRIDES, HETEROBORANES AND THEIR METALLADERIVATIVES] (Vol 4) Dicarba-closo-pentaborane(5)... 

Another structural class of the boron hydrides is the so-called quasi-closo boranes. They are related to the closo boranes by removing a framework atom from the latter and adding in its stead a pair of electrons. Thus one of the polyhedron framework sites is taken by an electron pair. 

Figure 3-19. Boron skeletons of boron hydrides. Left Regular icosahedral boron skeleton of B12H122 Right Closo, nido, and arachno boranes after Williams [38] and Rudolph [39], The genetic relationships are indicated by diagonal lines. Used with permission, copyright (1976) American Chemical Society.

A special case of molecular boron suboxides is found in the hydroxylated polyhedral boranes (see Boron Hydrides). For example, the icosahedral anion closo- Q 2H 2 and related compounds can be completely hydroxylated to form discrete molecular boron suboxides, including closo-[Bi2(OH)i2] These can be regarded as members of a class of formally hydrated suboxides related to compounds snch as tetrahydroxydiborane, B2(HO)4 (BO H2O). The terminal hydroxyl groups of these compounds can be fimctionahzed as esters or ethers to produce a variety of large molecule derivatives. 

All faces of the boron hydride polyhedra are equilateral or nearly equilateral triangles. Those boron hydrides that have a complete polyhedral shape are called closo boranes (the Greek closo meaning closed). One of the most symmetrical, and, accordingly, most stable, polyhedral boranes is the ion. Its regular icosahedral configuration is shown in Figure 3-26a. The structural systematics of closo boranes and related... 

There are boron hydrides in which one or more of the polyhedral sites is truly removed. Figure 3-26b shows the systematics of borane polyhedral fragments as obtained from closo boranes, after Williams [3-27] and Rudolph [3-24]. All the faces of the polyhedral skeletons are triangular, and thus the polyhedra may be termed deltahedra and the derived fragments deltahedral. The starting deltahedra are the tetrahedron, the trigonal bipyramid, the octa-... 

A nido (nestlike) boron hydride is derived from a closo borane by the removal of one skeleton atom. If the starting closo borane is not a regular polyhedron, then the atom removed is the one at a vertex with the highest connectivity. An arachno (weblike) boron hydride is derived from a closo... 

Imitation of carbon fullerenes In an analogy to carbon fullerenes, Lipscomb and co-workers proposed boron fullerenes as large closo boron hydride based on Descartes-Euler formula P + F=E- -2 P for vertices, Ffor faces. 

The best (and historically the first) understood are boron clusters, which class comprises boranes (poly-nuclear boron hydrides), carboranes (boranes with one or two B atoms replaces with C), and metallo-carboranes with a metal atom incorporated into the cluster. Their structures have been rationalized by Wade in a set of rules [238]. The structure of a boron cluster depends on the numbers of its skeleton atoms (u) and of skeleton electron pairs (SEPs)(p) available for bonding these atoms. If p = a- -l,the cluster is a closed deltahedron, i.e. a polyhedron having only triangular faces (closo structure). If p = a + 2, it adopts the form of a higher-order polyhedron with one... 

In Table I we list a few examples of the geometrical correspondence which prevails between closo boron hydrides and the carbon fiillerenes [3]. The geometrical structure of Buckminsterflillerene maps into that of the 32-vertex closo boron hydride, B32H32. Both molecules of symmetry Ih display correspondence of the geometrical centers of the 32 carbon (polygon) faces to the 32 boron vertices, the 60 boron faces to the 60 carbon vertices, and the 90 carbon contacts to the 90 boron contacts. In accordance with the Descartes-Euler formula, for both molecules the sum of the vertices and faces exceeds by 2 the number of contacts. A cursory review of Table I indicates that similar correspondences prevail for all the examples listed. Clearly the list of examples may be readily extended. 

We mention that the Descartes-Euler formula may be used to predict which closo boron hydride is the analog of each experimentally known fullerene. Thus, for a given fullerene, if the number of carbons is multiplied by 3/2 (a formal carbon contact number) to give the total number of carbon contacts, one may then calculate the number of carbon faces whose equality with the number of Won vertices yields immediately the molecular formula desired. 

There are various types of boron hydride and borohydride ion. Those with a closed polyhedron of boron atoms are designated closo compounds. Ones in which there is an incomplete polyhedron (by removal of one vertex) are called nido compounds (from the Greek word for nest ). Those with a more open structure (removal of two or more vertices) are arachno compounds (from the Greek spider ). 

The closo boron hydride anions have been studied considerably more than the open ions, nido and arachno. The reactions of the open ions are quite varied, and we will consider only a few of these before proceeding to the reactions of the closo anions. 

We have discussed certain boron hydride species such as the icosahedral carboranes and the closo boron hydride anions that are quite stable. A feature of these species compared to other stable boron compounds is that the fraction of their mass that is boron is very high. For example, 75% of the mass of the icosahedral carboranes (C2B10H12) is boron as is 69% of the disodium salt of the closo boron hydride anion B Hj In contrast, only 17% of the mass of boric acid is boron. Thus boron hydride species can deliver large concentrations of boron to situations where this is desired. 

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