Neutral carboranes

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). 

Ab initio computations have so far proved to be reliable structural tools for neutral carboranes and their anions. They have been used to predict the carborane geometries for six-vertex, eight-vertex, and 11-vertex nido-carboranes,67-69 and for 5-12 vertex r/aro-carboranes.70,71 However, carborane anions in the presence of a group I metal cation may form ion-pair anions with one cation tightly bound to the carborane anion (see Chapter 3.03).72,73 Even computations on a salt with a carborane anion in a non-coordinating solvent would need to include the cation for computed shifts that tally with observed NMR data.74... 

The polyhedra in Fig. 1 thus represent suitable shapes for cluster species with n skeletal atoms (each of which can furnish three AO s for use in skeletal bonding) and with (n + 1) skeletal bond pairs. Since it is the cluster symmetry that determines the number of bonding MO s, the same polyhedra can serve as the basis for the structures of a whole range of isoelectronic species, including neutral carboranes of formula C2B 2Hn, bismuth clusters, such as the trigonal-bipyramidal Bis " ",... 

Metallacarboranes have both metal and carbon atoms in the cage skeleton. In contrast to the metallaboranes, syntheses of metallacarboranes via low- or room-temperature metal insertion into carborane anions in solution are more controllable, usually occurring at a well-defined QB, open face and forming a single isomer. Other preparative routes, such as metal insertion into neutral carboranes at high temperature, involve more complex processes and tend to give less clean-cut results, and such approaches are rarely employed today. Typical syntheses that are based on readily available 11-vertex and 7-vertex nido-carborane ligands are as shown ... 

Although the existence of C2B10Hi22 ions derived from the o-, m- and p-carboranes were reported already during the 1960 ies1 4, their number and their structures have remained largely unknown. The chemistry of the dianions has been extensively explored, but they have not yet been isolated as salts. In the present study we discuss the uptake of one and two electrons by each of the three carboranes, based on ab initio calculations, and present the resulting optimized structures and relative stabilities of the neutral carboranes and their anions. 

Because of their completely deltahedral pseudospherical shapes, the dianions, the singly charged monocarbaborane anions [CB iH ], and the neutral carboranes of formulae C2B 2H are known as closo (closed cage) clusters. Their skeletal shapes are shown in the left-hand column of Figure 3.1. 

The relationship of nido and arachno boranes and carboranes to notional parent closo boranes and carboranes, by removal of BH units from selected sites, usually of high connectivity, can be illustrated simply by exploring how the MOs of a closo system B,I I, (or isoelectronic neutral carborane C2B, H ) will be affected by the theoretical experiment of removing one or two... 

Novel and highly active RCM catalysts that contain anionic or neutral carborane tags have been developed. Their uses in promoting standard RCM in CH2CI2 at 30° have been demonstrated. ... 

Since the number of bonding molecular orbitals is determined by the cluster symmetry, the same polyhedron can serve as the basis for the structures of an ample range of isoelectronic species as neutral carboranes with formula C2B -2H or the naked clusters as Ge that will be dealt with in Chapter 4. 

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