Polyhedral borane

Cluster-expansion and cluster-degradation reactions are a feature of many polyhedral borane species. Examples of cluster-expansion are " ... 

Decaborane is the most studied of all the polyhedral boranes and at one time (mid-1950s) was manufactured on a multitonne scale in the USA as a potential high-energy fuel. It is now obtainable in research quantities by the pyrolysis of B2H9 at 100-200°C in the presence of catalytic amounts of Lewis bases such as Me20. B10H14 is a colourless, volatile, crystalline solid (see Table 6.2, p. 163) which... 

As a result of the systematic application of coordination-chemistry principles, dozens of previously unsuspected stnicture types have been synthesized in which polyhedral boranes or their anions can be considered to act as ligands which donate electron density to metal centres, thereby forming novel metallaboranc elusters, ". Some 40 metals have been found to act as acceptors in this way (see also p. 178). The ideas have been particularly helpful m emphasizing the close interconnection between several previously separated branches of chemistry, notably boron hydride clu.ster chemistry, metallaboranc and metallacarbaborane chemistry (pp. 189-95). organometallic chemistry and metal-metal cluster chemistry. All are now seen to be parts of a coherent whole. 

E. L. Meutterties and W. H. Knoth, Polyhedral Boranes, Marcel Dekker, New York, 1968, 197 pp. 

The chemistry of polyhedral borane ions. L. J. Todd, Prog. Boron Chem., 1970,2,1-35 (81). 

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. 

The area of chemistry involving the polyhedral boranes and carboranes has seen enormous growth in recent years. Accordingly, the brief survey here does not include many interesting facets of these fascinating compounds. For more details on the polyhedral boranes, the references at the end of this chapter should be consulted. 

Heteroatom Cluster Compounds Incorporating Polyhedral Boranes as Ligands... 

This, together with the Ir-H terminal bond indicates that the compound can be regarded as a further example of the growing number of compounds in which the high oxidation state Ir(v) is stabilized by coordination to a soft polyhedral borane ligand... 

This mechanism bears a close relationship to the polymerization of polyhedral boranes. Certainly there is clear evidence for the formation of Fe(CO)4 from Fe(CO)5. The production of Fe2(CO)9 from Fe(CO)5 is catalyzed by light, but further polymerization of Fe2(CO)9 to Fe3CO)I2 is not observed under these conditions. For the related osmium systems, there is some evidence for the formation of 082(00)9 from Os(CO)5, but irradiation of Os3(CO)J2 does not produce higher clusters although in the presence of molecules such as CO or CjH L) the compounds... 

The polyhedral boranes and carboranes discussed above may be regarded as boron clusters in which the single external orbital of each vertex atom helps to bind an external hydrogen or other monovalent atom or group. Post-transition main group elements are known to form clusters without external ligands bound to the vertex atoms. Such species are called bare metal clusters for convenience. Anionic bare metal clusters were first observed by Zintl and co-workers in the 1930s [2-5], The first evidence for anionic clusters of post-transition metals such as tin, lead, antimony, and bismuth was obtained by potentiometric titrations with alkali metals in liquid ammonia. Consequently, such anionic post-transition metal clusters are often called Zintl phases. 

Such equations vaguely resemble Lipscomb s equations of balance for the polyhedral boranes [Eqs. (1) and (2)]. 

Rules for counting the number of skeletal electrons provided by each vertex atom need to be established in order to determine the number of skeletal electrons in polygonal and polyhedral clusters of the post-transition elements. The rules discussed above for polyhedral boranes can be adapted to bare post-transition metal vertices as follows ... 

Today the chemistry of diborane and the polyboranes is well understood [2] and much of it is textbook knowledge. Therefore, after a brief survey, emphasis will focus on the development of polyhedral borane chemistry within recent decades, and even restricting discussions to homopolyboranes only certain areas can be dealt with. This incorporates synthetic procedures, the chemistry of some polyboranes and particularly polyborane anions. Other chapters of this book are devoted to heteropolyboranes such as the carbaboranes (see Chapter 3.1), azaboranes and related heteropolyboranes (see Chapter 3.3) of the main group elements. In these areas enormous progress has been achieved within the last two decades. 

Another very useful rule for classifying the structures of polyboranes and hetero-boranes as well as many metal boron cluster compounds and their derivatives has been developed by Rudolph, Williams, Mingos and Wade (see Chapter 1.1.2) [4]. Today these are generally termed the Wade rules. They can be derived from the structures and electronic requirements of closed polyhedral boranes, such as an octahedron or an icosahedron, which are present in the anions B6H62 and B,2 H, 22. Since there are only exopolyhedral B-H bonds the number of electron... 

In addition to these polyhedral boranes there exist polyboranes where two (or more) polyboranes are connected by single B-B bonds or by sharing three or four boron atoms of two (or more) borane units by 3c2e bonds. In the case of B10H16 there are two B5H8 units where the apical boron atoms of B5H9 are joined by a B-B bond (l,l -isomer) or by a B-B bond between the apical boron atom of one pentaborane unit with a basal boron atom of a second pentaborane fragment... 

Geometrically it is impossible for polyhedral boranes where two closo-polyboranes share two common boron atoms, i.e., a common edge, to exist, because the hydrogen atoms of the neighboring BH groups would interfere sterically. For instance, if two B12H12 units are joined by a common edge this would result in H-H distances of less than 1.5 A, well below the van der Waals contacts. However,... 

In contrast to the polyhedral boranes B H +m there exist a number of neutral boron duster molecules B X (X = Cl, Br, I, NR2, R) all of them having closed deltahedral structures in spite of the fact that the number of bonding electron pairs is only n. For this reason these homonudear cluster compounds of boron are called hypercloso polyboranes. However, there also exist anions of type B X 2 which fit Wade s rules. 

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