Boranes and carboranes

Placement of Extra Hydrogens. The placement of extra hydrogens plays a cmcial role ia determining the stmctures adopted by boranes and carboranes. However, the exact position of extra hydrogens sometimes depends on the physical state of the molecule, eg, the... 

Neutral carboranes and boranes react with transition-metal complexes forming metallocarboranes or metalloboranes, respectively. However, most metallocarboranes and metalloboranes are prepared from transition-metal halides and anionic carborane and borane species ( 6.5.3.4) or by reacting metal atoms and neutral boranes and carboranes. These reactions are oxidative addition reactions ( 6.5.3.3). 

L Monomeric and Polymeric Organic Analogs of Boron Cluster Systems The polyhedral clusters of boranes and carboranes are groups of cluster systems that are present ubiquitously in organoboron polymers. As in the case with the... 

Several dendrimeric borane and carborane systems have been developed for use as boron-delivery agents.147 In vitro tests with a boronated dendrimer-epidermal growth factor bioconjugates indicated that they are endocytosed, resulting in the accumulation of boron in lysosomes. 

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. 

Chapters in the book The Borane, Carborane, Carbocation Continuum are of interest Patterns of structures in boranes and carboranes, The carboranecarbocation continuum, Untangling molecular structures, and New species of boranes and carboranes. ... 

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. 

The same polyhedra serve as the basis for the structures of nido and arachno compounds, too, although for these boranes and carboranes the polyhedra are incomplete. Nido structures are adopted by neutral boranes BjjHw j.4, carboranes CBy 4Hn.. 3, CsBji—... 

The positions of the hydrogen atoms in boranes and carboranes are worth noting. In doso-borane anions the n BH units are aligned... 

III. Bonding In Boranes and Carboranes A. Localized Bond Treatments... 

Although the structural pattern outlined in the foregoing can be rationalized at a simple quabtative level by using a molecular orbital approach to the skeletal bonding of boranes and carboranes (see Section III, B) it is useful to consider first what problems are encountered if one attempts to describe the bonding in terms of localized bonds. 

The problems outlined in the previous section can be avoided if, instead of allocating the skeletal bonding electron pairs to localized bonds, one simply compares their number with the number of skeletal bonding MO s (199). The closo, nido, and arachno structures of boranes and carboranes can then be seen to reflect the numbers of skeletal bond pairs that are available to hold their skeletal atoms together. 

The structures of nido- and amcAwo-boranes and carboranes can be rationalized in a similar manner to those of the closo species if one considers the hypothetical anions, and B Hn , from which they... 

The triangular-faced polyhedra shown in Fig. 1 form the bases for the structures of boranes and carboranes. 

That transition metal-carbonyl clusters, which contain an apparent abundance of electrons, might have much in common with boranes and carboranes, notorious for their deficiency of electrons, appears at first sight improbable. However, the structural and bonding relationship between them becomes apparent if one considers certain metal-carbonyl clusters for which localized bond treatments are unsatisfactory. 

Large numbers of metallo-boranes and -carboranes (31h, 68a, 94a, 95, 96a, 96b, 172a, 183, 195, 205b, also Williams, this volume) are now... 

AO s and 2 electrons for cluster bonding, so it is not surprising to find such units in cloao-metallo-boranes and -carboranes. 

Benzvalene, CgHe, and the carbo cations C5H5+ and CeMee have already been cited (Section VI) as hydrocarbons of the same structural type as boranes and carboranes, and we have also seen that such aromatic... 

The suggested rules having been itemized in the previous sections, a chemical rationalization of thedr origins follows, after which the various types and families of boranes and carboranes are discussed in detail. 

To understand the unusual structures found in bare post-transition element clusters and underlying reasons for differences in the structures between analogous boranes and post-transition element clusters, we carried out extensive density functional theory (DFT) studies of bare germanium clusters during the past decade [18-20, 69-72]. In three cases (Fig. 5), these studies predict lowest energy structures for Ge clusters that are different from the well-established structures of the isovalent boranes and carboranes C2B 2H . Thus, the lowest energy... 

Molecular Orbital Descriptions. In addition to the localized bond descriptions, molecular orbital (MO) descriptions of bonding in boranes and carboranes have been developed (4). Early work on boranes helped develop one of the most widely applicable approximate MO methods, the extended Htckel method. Molecular orbital descriptions are particulady useful for c/oso molecules where localized bond descriptions become cumbersome because of the large number of resonance structures that do not accurately reflect molecular symmetry. Such descriptions show that the highest occupied MO (HOMO) is degenerate for most deltahedral B H molecules, but that a closed shell is obtained for the corresponding [B H l2 anions. After... 

---