Five BORON COMPOUNDS

It is thus possible to distinguish five types of boron compound, each having its own chemical systematics which can be rationalized in terms of the type of bonding involved, and each resulting in highly individualistic structures and chemical reactions ... 

Boron and carbon are consecutive first-row elements. Consequently, pentacoordi-nate monopositively charged carbonium ions are isoelectronic with the corresponding neutral pentacoordinate boron compounds that is, the BH5 molecule is isoelectronic with CH5+. Within this context, it is interesting to refer to the five-coordinate... 

A large number of boron compounds are known in which the boron atoms are arranged in the form of some sort of polyhedron (octahedron, square antiprism, bicapped square antiprism, icosahedron, etc.). In these structures, the boron atoms are most often bonded to four, five, or six other atoms. The most common structure of this type is the icosahedron shown by the species Bi2Hi22. This structure can be considered as a B12 icosahedron with a hydrogen... 

The metal borides are one of the five major classes of boron compounds (1). In the following we review the geometric and electronic structural data with an emphasis on the transition metal borides. Because the structures of transition metals and elemental boron provide end points, we begin by reviewing the solid state structures of these elements. A brief survey of the range of metal boride structures in general is followed by some more detailed consideration of the problems of electronic structure raised by the geometries of the transition metal borides. 

The SjSii ring system is susceptible to attack by nucleophiles, as is illustrated for 636. Examples of nucleophiles are alcohols (best with primary and secondary), 1563-1538 phenols,hydroxide ion, water, " and carboxylic acids. The boron compound 641 reacts with 636 to give the five-membered heterocycle 642. The electrophilic reagents, acetyl chloride and benzoyl chloride, react with tetraphenylcyclodisiladithianes to give the diacyl sulfide and dichlorophenyl-silane. ... 

As already emphasized in Chapter 3.17.3.2.2, "B NMR spectroscopy is an essential tool in understanding the electronic environment in boron compounds. Again, some compounds of five-membered rings with an additional nonadjacent heteroatom is mentioned as representative of many other examples. 

Aluminum has 3d orbitals relatively accessible, and not only may the valency of aluminum rise above four, but some d character may be present in the bonds of the tetravalent and also in the bonds of the trivalent aluminum compounds. At present only few organic aluminum compounds with five- and six-coordinated aluminum are known (sp3d and sp3d2 hybrids see Sections III,D and IV,C). The differences between the behavior of aluminum and boron compounds can partially be explained by the possibility of formation of these structures (trigonal bipyramid, octahedron). 

This review, concerned with reactivity of the boranes with no attempt to make the references exhaustive, has demonstrated the current and intense activity in this area of chemistry. The reader may already have recognized that nearly 60% of the references quoted refer to work published in this field during the last five years. With the application of new methods of preparation and separation, it is evident that boron compounds will be described in increasing numbers. Meanwhile, it is hoped that the subject matter reviewed here will be of help to research workers and teachers alike. 

The various stoichiometries are not equally common, as can be seen from Fig. 6.5 the most frequently occurring are M2B, MB, MB2, MB4 and MBfi, and these five classes account for 75% of the compounds. At the other extreme RunBg is the only known example of this stoichiometry. Metal-rich borides tend to be formed by the transition elements whereas the boron-rich borides are characteristic of the more electropositive elements in Groups 1-3, the lanthanides and the actinides. Only the diborides MB2 are common to both classes. 

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