Carboranes bridge hydrogen

Parent carboranes are composed of carbon, boron and hydrogen atoms. Whenever a hydrogen bridge occurs (BHB or CHB), the electron of the bridging hydrogen atom is needed for the framework bonding of the cluster. Replacement of the... 

By no means do all metallocarboranes have the metal atoms occupying vertices of the basic polyhedra. Apart from many derivatives in which o-bonded metal residues occupy exo sites attached to particular skeletal atoms, several metalloboranes and -carboranes are known in which the metal occupies an edge-bridging site, effectively replacing a bridging hydrogen atom of the parent borane. Many are nido species related to BeHio, for example, the /x-silyl and /i-germyl carboranes. 

Of the atoms within the carboranes, the bridge hydrogens (rule 2) are in the most alien environments (as compared to the common one-coordinate terminal hydrogens). Most bridge hydrogens are 100% over their normal coordination environment (two-coordinate), whereas some are 200% over normal (three-coordinate). 

Onak s CB5H7 (III-C4) 101) is under investigation by Beaudet 3), who has determined that the lone 666-bridge hydrogen is almost certainly above a triangular face. This undesirable situation (a violation of rule 2p) probably accounts for its instability, i.e., in the presence of a Lewis base (proton acceptor) the related closo-anion CBsHe" is produced 116). More recently, R. R. Rietz has reported (private communication, 1974) a second closo-carborane, CBgHg. 

Almost all of the ido-carboranes have bridge hydrogens, and the organization of their structures is much more complicated than closo-carborane structures because the bridge hydrogens (rule 2) apparently... 

Among the wido-carboranes there are isomers wherein the carbons speciously outrank bridge hydrogens, but they appear to be less stable than isomers that allow the bridge hydrogens to occupy the optimal sites (see rules 2s and 3s in Tables III and IV). 

When carborane analogs of B gH x 1 are discovered, it would be expected that an isomer of CB4H10 such as IV-A15 (violating only rule 3s) would be more stable than the isomer III-A15 which contains two undesirable 6 6-bridge hydrogens. Near the completion of this manuscript, Matteson and Mattschei reported (91) a CB Hxo isomer for which they suggest the IV-A15 structure. 

In Table VII are illustrated, in greatly oversimplified fashion, the relationships of the known boranes and some carboranes. The lack of acidity of BgHg (8) may be attributed to both its small size and 5 5 -bridge hydrogens. 

Carboranes, 18 2, see also specific types analogs and derivatives, 18 97-132 BH group-substituted, 18 78 CNPR theory, 18 125-127 bond distances in, 18 46 bonding in, 18 7-16 bridge hydrogen, 18 90-93... 

Lowry-Bronsted acidity of bridge hydrogens, 18 135 of carboranes, 18 132-136 Low-temperature condensation, of high-temperature species, 14 121-171 activation enthalpy in, 14 128-129 atomic species in, 14 123-125 condensation process in, 14 129-130 experimental methods in, 14 130-141 formation of high temperature species, 14 131-139... 

Fig. 1. Structures and numbering of the three isomeric icosahedral carboranes, and the degradation of 1,2-C.2BioHi2 to 7,8-C2B9Hir. The bridging hydrogen is shown in one of the two equivalent bridging positions.

The unsubstituted nido-carborane C2B7Hn exhibits an NMR spectrum indicating a structure similar to that of the dimethyl species, in terms of vertex positions, but the hydrogen positions are different133 . Apparently there is a BH2 group and only one bridging hydrogen. 

The skeletal electron contribution of a BH unit is clearly 2, since 2 of the 4 valence electrons are required for the B—H bond. Similarly, CH groups (as in carboranes, see below) are 3-electron donors. B—H—B bridging hydrogen atoms and extra terminal hydrogen atoms (as in BH2 units) contribute 1 electron each. 

Similar bridging hydrogen atoms occur in many other boranes, as well as in carboranes, which contain both boron and carbon atoms arranged in clusters. In addition, bridging hydrogens and alkyl groups are frequently encountered in aluminum chemistry. A few examples of these compounds are shown in Figure 8-15. 

Figure 3.10. Localized bond schemes for square pyramidal and pentagonal pyramidal carboranes, showing only one of the several possible canonical forms in each case. (2,4-C2B4H8 and 2,3,5-6363117 incorporate CHB bridge hydrogens. Relative energies, jei in kcal per mole.)...

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