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Boron Hydrides 309

FIGURE 10 The structures of two nido (left) and one arachno carboranes. The small filled circles represent hydrogen
atoms. [From Onak, T. (1975). In “Boron Hydride Chemistry” (E. L. Muetterties, ed.), p. 353, Academic Press, New
York, Figures 10.3 and 10.4.]

nine nido and arachno carboranes and methyl substituted explored for the production of high-temperature resins
carboranes. and elastomers and have found utility for the stationary
Closo carboranes can be formally constructed from phases of gas-phase-chromatography columns.
closo boron hydride anions by exchanging two boron Preparing ortho-carborane is the first step in the prepa-
atoms and two negative charges for two carbon atoms. ration of the icosahedral carboranes. This compound is
Thus, the two most important closo anions, B 10 H 2− and made in high yield by a two-step process in which acety-
10
2−
B 12 H , are isoelectronic and isostructural with C 2 B 8 H 10 lene is inserted into the nido cage of B 10 H 14 . In the first
12
and C 2 B 10 H 12 , the former having the geometry of a very step B 10 H 14 is reacted with a Lewis base (L), most usually
slightly distorted bicapped square antiprism and the latter diethyl sulfide [(CH 3 CH 2 ) 2 S].
having the geometry of a very slightly distorted icosahe-
dron. The slight distortions in these compounds from the B 10 H 14 + 2L −→ B 10 H 12 L 2 + H 2
perfect symmetry of the boron hydride anions are the re- Gaseous acetylene is then passed through a solution of the
sult of the differences in the lengths of bonds formed by B 10 H 14 -Lewis base derivative giving the desired ortho-
boron compared to those formed by carbon. carborane product.
Of all of the carboranes, by far the most studied are the
icosahedral species, C 2 B 10 H 12 . As seen in Fig. 11, three B 10 H 12 L 2 + HCCH −→ o-C 2 B 10 H 12
isomers exist. Their systematic names are 1,2-dicarba-
It is worth noting that although B 10 H 14 has an n + 4 elec-
closo-dodecaborane(12), 1,7-dicarba-closo-dodecabo-
tron count and thus should be the result of 1 vertex being
rane(12), and 1-12-dicarba-closo-dodecaborane(12), but
removed from an 11-vertex deltahedron, in this reaction it
they are generally known as ortho-carborane, meta- adds two vertices and forms the much more stable icosa-
carborane, and para-carborane, respectively. They are
hedron.
very stable compounds with many known chemical
Ortho-carborane is converted to meta-carborane in 98%
reactions and derivatives. Their derivatives have been
conversion by passage of the vapor through a tube held at
◦
600 C with a residence time of less than a minute. Thermal
conversion of meta-to para-carborane occurs at 700 C,
◦
but the product is a mixture of 75% meta-carborane and
25% para-carborane.
V. REACTIONS OF BORON HYDRIDES

A. Combustion

FIGURE 11 The structures and numbering conventions of the The great stability of the boron-oxygen bond and the rela-
three isomers of the icosahedral carboranes. Radially directed, tiveinstabilityoftheboronhydridessuggeststhatthecom-
terminal hydrogen atoms at each boron and carbon atom have bustion of a boron hydride should be a highly exothermic
been omitted. [From Beall, H. (1975). In “Boron Hydride Chem-
istry” (E. L. Muetterties, ed.), p. 302, Academic Press, New York, process. In fact, the boron-oxygen bond has a dissociation
Figure 9.1.] enthalpy of about 800 kJ/mol, and pentaborane(9), one of