Boron Hydride Cages

The boron hydrides are one of most beautiful classes of polyhedral compounds whose representatives range from simple to rather complicated systems. Our description here is purely phenomenological. Only in passing is reference made to the relationship of the characteristic polyhedral cage arrangements of the boron hydrides and the peculiarities of multicenter bonding that has special importance for their structures. 

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. 

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 

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The basic principles of the structure and naming of boron hydride cage compounds have been discussed in Chapter 16. See particularly Fig 16.50. A very similar figure is presented in the Red Book with a similar explanation. The names of die structure types are sutnma nzcd in Table J-8J. 

FIGURE 9.2 Some boron hydride cage compounds. 

This chapter concerns carboranes (carbaboranes), which are boron clusters with at least one carbon atom as part of the polyhedral cage. Published studies on carboranes before 1981 were reviewed in GOMC (1982) and between 1982 and 1992 in COMC (1995). The present review covers the period of 1992 to early 2005. Unlike in previous chapters, boron hydrides with organic substituents attached to a boron atom, organopolyboron hydrides, are not discussed in this chapter. Borane clusters containing at least one non-carbon atom as part of the cage framework are reviewed in Chapters 3.03, 3.04 and 3.05 of this volume. 

CARBORANE. A cry stalline compound composed of boron, carbon, and hydrogen. It can be synthesized in various ways, chiefly by the reaction of a borane (penta-or deca-) with acetylene, either at high temperature in the gas phase or in the presence of a Lewis base. Alkylated derivatives have been prepared. Carborancs have different structural and chemical characteristics and should not be conTused with hydrocarbon derivatives or boron hydrides. The predominant structures arc the cage type, the nest type, and the web type, these terms being descriptive of the arrangement of atoms in the crystals. Active research on cargorane chemistry has been conducted under sponsorship of the U.S. Office of Naval Research, http //www.onr.navy.mil/... 

Fig. 22. The structure of the unusual mixed metal boron hydride complex Fe(CO)3(BsH8)Cu-(PPh3)2, whose FeBs cage is based on BgHio (Ref. 113)...

A zwitterionic [RNHj]" [CB,oH,2] obtained from the action of alkylisocyanides on decaborane(14) is a cage insertion of one carbon atom into a borone hydride framework to give another compound (as a nitrogen derivative) in the nido-carborane series, CB H +4 ... 

Numerous reactions between phosphines and the boron hydrides are now known. Some of these reactions involve the breaking of B-H bonds with the elimination of hydrogen (9.85, 9.86), while others involve the break-up of the complex boron cage into smaller units (9.87-9.90). Non-phosphorus ligands of basic character also form complexes in this latter way [35]. 

Closo boron hydride clusters are anions with the general formula [BnH ] that have the structure of a polyhedron with triangular faces [59]. They are very stable and unreactive and because of that they are considered aromatic. They obey the 2n + 2 Wade rule [17, 60], in which n are the vertexes of the polyhedron, or Mingos 4n + 2 rule [16, 61]. Both rules are equivalent Wade s rule refers to the cage electron pairs whereas Mingos rule incorporates also the exo electron pairs corresponding to the B-H bonds, thus referring to the total number of valence electrons. 

Kaczmarczyk, A. Kolski, G. B. Polarizability of the closed-cage boron hydride BjoKfo, J- Phys. Chem. 1964, 68, 1227-1229. 

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