Boranes cluster compounds

Boranes, boron clusters, and in particular, carboranes are of special interest due to their unique properties that cannot be found in organic counterparts. These uniqne properties are based either on the element boron, due to its electron deficiency, or on the structnral featnre of the cluster compound. Borane clusters as a class of materials have a wide range of potential applications. This is not only due to their unique electronic and nuclear features the fields of application, to name but a few, range from materials science through medical applications to catalysis, which will be described in more detail below [13]. Carboranes can be applied as liquid crystals in electro-optical displays [14], non-linear optics [15], and ion-selective electrodes [16] in the materials science arena. If carboranes are vaporized and fired at high temperatures they create boron films that are applied in Tokamak reactors for nuclear fusion [17]. Boranes have furthermore found application in airbag propellant systems in cars [18], as the stationary phase in gas chromatography [19] and in metal ion extraction systems, for example, for nuclear waste [20]. In medical applications, boron neutron capture therapy (BNCT), a special field of anti-cancer therapy, is noteworthy. 

Heteroatom Cluster Compounds Incorporating Polyhedral Boranes as Ligands... 

Even though qualitative bonding descriptions of metal atom clusters up to six or seven atoms can be derived and in some cases correlated with structural detail, it is clear that most structures observed for higher clusters cannot be treated thus. Nor do the structures observed correlate with those observed for borane derivatives with the same number of vertices. Much of borane chemistry is dominated by the tendency to form structures derived from the icosahedron found in elemental boron. However, elemental transition metals possess either a close-packed or body-centered cubic arrangement. In this connection, one can find the vast majority of metal polyhedra in carbonyl cluster compounds within close-packed geometries, particularly hexagonal close-packing. 

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 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. 

The geometries of metal carbonyl and metallocarborane cluster compounds have been systematized recently by a set of simple rules described collectively as the polyhedral skeletal electron pair theory (153, 218, 232). This approach originated from a perceptive analogy between isostructural metal carbonyl and borane polyhedral cluster molecules (232), and its applications have been widely discussed and reviewed (147, 153, 210, 218, 233, 234, 235, 240). In this review,... 

The boron hydrides, including the polyhedral boranes, heteroboranes, and their metaUa derivatives, encompass an amazingly diverse area of chemistry. This class contains the most extensive array of structurally characterized cluster compounds known. Included here are many novel clusters possessing idealized molecular geometries ranging over every point group symmetry from identity (C[) to icosahedral (I[). Because boron hydride clusters may be considered in some respects to be progenitorial models of metal clusters, their development has provided a framework for the development of cluster chemistry in... 

Another class of molecules that will be discussed contains cluster compounds such as the polyhedral borane anions, B H -", and some metal containing species such as the metal carbonyl clusters. 

Dibromo(p-chlorophenyl)borane, as catalyst, 9, 223 Dibromoethynes, in iron cluster compounds, 6, 296 (Dibromomethyl)methyldiphenylsilane, in Grignard reagent preparation, 9, 40... 

Since polynuclear complexes and cluster compounds are in general rather complicated species, the application of quantitative methods for describing bonding is not only difficult but also impractical. Qualitative approaches and empirical rules often play an important role in treating such cases. We have used the octet rule and bond valence to describe the structure and bonding of boranes and their derivatives (Sections 13.3 and 13.4). Now we use the 18-electron rule and bond valence to discuss the bonding and structure of polynuclear transition-metal complexes and clusters. 

K. Wade, The Structural Significance of the Number of Skeletal Bonding Electron-pairs in Carboranes, the Higher Borane Anions, and Various Transition-metal Carbonyl Cluster Compounds, Chem. Comm. 1971, 792-793. 

S. Kher and J.T. Spencer, The Chemical Vapor Deposition of Pure Nickel and Nickel Boride Films from Borane Cluster Compounds, Proceedings of Material Research Society Symposium, Vol.250, 1992, pp.311-316. 

There are many examples of borohydride compounds of these metals, e.g., Cu, Ag, Zn and Cd-BH as neutral and anionic complexes in which the mode of bonding of BH is dependent on the coordination number of the metaP. Higher borane anions also combine with Cu and Ag, yielding both neutral and anionic complexes. Although no borohydrides of Au are isolated, treatment of Au-halide complexes with, e.g., NaBH, is a standard method for the preparation of Au-cluster compounds Copper(I) hydride, first reported in 1844, has the ZnS structure [d(Cn-H) = 0.173 nm (1.73 A) d(Cu-Cu) = 0.289 nm (2.89 A)] and decomposes to the elements when heated. At >100°C the decomposition is explosive. 

The boranes, carboranes, and related compounds are also of interest in the field of cluster chemistry, the chemistry of compounds containing metal-metal bonds. The bonding in these compounds will be discussed and compared with the bonding in transition metal cluster compounds in Chapter 15. 

The structures of several carbonyl cluster compounds were shown in Chapter 13. Many carbonyl clusters have stmctures similar to boranes it is therefore of interest to determine to what extent the approach used to describe bonding in boranes may also be applicable to bonding in carbonyl clusters and other clusters. 

Cage structures range from clathrate compounds on the one hand to metal-metal clusters and boranes on the other. These classes are discu.ssed elsewhere, " and this section will be restricted to certain nonmetal compounds having cage structures. 

Cluihralc compounds arc discus.scd both in Chapter 8 and earlier in this chapter, while metal clusters and boranes arc found later in this chapter. 

There are some interesting observations too concerning the structures of polyhedral molecules. Very often they are electron deficient in the sense that there are fewer than two electrons for each close contact. The heavy atoms forming the skeleton of the molecule may be either main group atoms (for example in the boranes and carboranes) or transition metal atoms (metal cluster compounds) or both (metallocarboranes). 50 shows the structures expected from Wade s rules for five atom polyhedral molecules with six, seven and eight pairs of skeletal electrons. There are a total of fifteen skeletal orbitals... 

Multinuclear Cluster Compounds Containing o-Borane Ligands. 138... 

Multinudear Cluster Compounds Containing or-Borane Ligands... 

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