Carborane-metal complexes, with

Besides transition metal complexes with weakly coordinating cteo-borate anions, metallacarboranes in which metal atom incorporated into carborane cage are of great interest... 

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). 

Precise placement of metal complexing sites within the infrastructure of a cascade molecule is of importance from a variety of perspectives. In the construction of the above noted Micellane family (cf. Sect. 3.1), we reported the construction of dendrimers with four alkyne moieties at sites equidistant from each other in the interior (17, Fig. 8) [60]. These were treated with decaborane (B10H14) to afford 1,2-dicarba-closo-dodecaboranes (o-carboranes) [71]. Rendering boron clusters soluble in water is of interest because of their use in cancer treatment by Boron Neutron Cancer Therapy. First and second generation water-soluble dendrimers containing four and twelve precisely located boron cluster sites, respectively, were synthesized (e.g., 18). These water soluble dendrimers and their precursors were characterized by 13C-, and nB-NMR spectroscopy (Fig. 8). 

The stability of the liquid carboranes depends on the substituents R at carbon and boron. The axial (endo) hydrogen atom is acidic and involved in 3c2e bonding to one of the basal boron atoms. In the 1H NMR it exhibits a high field shift near S = —1.4. Deprotonation of 55 with potassium or Bu Li leads to the anion (55-H)-, which is isolobal with C5H5. Reactions of 55 and (55-H) with appropriate metal complexes lead to metallacarboranes with sandwich structures [67, 69],... 

The material reviewed in this Chapter hitherto has focused on metallacarboranes in which the metal atom is a vertex in an icosahedral cage framework. Until recently, monocarbollide metal compounds with core structures other than 12 vertexes were very rare since suitable carborane precursors were not readily available." However, Brellochs recent development of the reaction of decaborane with aldehydes to give 10-vertex monocarboranes permits a considerable expansion in this area of boron cluster chemistry. As a consequence, several intermediate-sized monocarboranes are now easily accessible and we have recently begun to exploit the opportunities that these present. In particular, we have focused thus far on complexes derived from the C-phenyl-substituted species [6-Ph- zJo-6-CBgHii] It is clear from these initial studies that a wealth of new chemistry remains to be discovered in this area, not only from among the metal derivatives of PhCBg car-boranes such as those discussed in this section, but also in the metal complexes of other newly available carboranes. 

In the early 1960s it was recognized (6,187) that there were bonding similarities between the pentagonal face of the isomeric [nirio-(y HjJ2- ions and the well-known cyclopentadienide ion (Cp ) [CydJ- (Fig. 16). The isomeric nido-C2B9H1 J2- ions, which are commonly known as dicarbollide ions, and many other carborane anions, form stable complexes with most of the metallic elements. Indeed nearly all metals can be combined with polyborane hydride clusters to produce an apparently limitless variety of cluster compounds. 

In addition to that said above, complexes with closopolyhedral hydroborate metal-containing fragments are known, for example 564, obtained through the interaction of tetramethylammonium salt of nido-B9H12 anion and nickelocene [957], An up-to-date covering of carborane and metallacarborane structures and properties is given in Ref. 184f. 

The electronic spectrum is yet another property which illustrates the similarities between the metallocenes and (7r-ollyl) metal compounds. In Table VI are listed some data for a series of Coin(absorption bands with the small extinction coefficients are probably two of the spin-allowed d-d transitions. Scott (34) has developed an approximate axial ligand field model for the carborane-transition metal complexes and has discussed the optical spectra in relation to this bonding theory. The actual assessment of bonding in the (7r-ollyl) metal compound as well as the metallocenes would be greatly aided by accurate assignments of the electronic spectra. 

Spencer, Green, and Stone (88) have recently described a new synthetic approach to metallocarboranes in which an organomctallic transition metal complex is thermally reacted with a neutral cZoso-carborane, resulting in the incorporation of the metal into the polyhedron ... 

Reaction of the mdo-carborane monoanion 7,8-C2B9Hi2 with phosphine-substituted transition metal complexes afforded novel hydridophosphino-mctallocarboranes (80) according to... 

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