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Boron Hydrides 313

atoms in B 10 H 2− have substitutional reactivity similar to
10
all of the boron atoms in B 12 H , but the apical boron
2−
12
atoms in B 10 H 2− are more susceptible to some substitu-
10
tions than the equatorial and less susceptible to others.
[H 3 O] 2 B 10 H 10 + CH 3 CN −→ B 10 H 9 NCCH 3 (apical)
[H 3 O] 2 B 10 H 10 + NH 2 C(O)H −→ B 10 H 9 OC(NH 2 )H
(equatorial)

VII. REACTIONS OF CARBORANES
FIGURE 16 The structure and numbering system for the boron
atoms in B 10 H 14 . [From Lipscomb, W. N. (1975). In “Boron Hydride A. Nido Carboranes
Chemistry” (E. L. Muetterties, ed.), p. 57, Academic Press, New
York.] Electrophilic substitutions can be performed at the boron
atoms of nido carboranes as they can on the boron hy-
drides. Charge distributions have been calculated for these
strong electrolytes. The anions are generally isolated from compounds and electrophilic substitution occurs, as ex-
solution as hydrates. Most dipositive cations such as Sr 2+ pected, on boron atoms holding the greatest negative
and Fe 2+ do the same. Large unipositive cations such Tl + charge. The hydrogen atoms attached to carbon atoms in
and (CH 3 ) 4 N form salts that are only slightly soluble in the carborane cage are weakly acidic and can be abstracted
+
water. by a very strong base such as butyllithium. Metathesis re-
A large number of reactions in which H atoms are re- actions with halogen compounds can then produce deriva-
placed by other substitutents are known, and the stability tives in which the carborane framework is unchanged, and
of the B 10 H 2− and B 12 H 2− cages allows reactions under
10 12 the new substituents are attached to the carbon atom from
quite rigorous conditions to proceed with little or no cage which the proton was removed, for example,
degradation. Substitution reactions can be performed with
+
B 10 H 2− and B 12 H 2− salts or with H 3 O counterions, i.e., 1,6–C 2 B 4 H 6 + 2BuLi −→ 1,6–Li 2 C 2 B 4 H 4
10
12
the acid form of the anion. Such acid catalysis often leads + 2BuH (Bu = C 4 H 9 )
to facile reactions.
The acid forms of each of these closo anions can be fully 1,6–Li 2 C 2 B 4 H 4 + 2CH 3 I −→ 1,6–(CH 3 ) 2 C 2 B 4 H 4
deuterated in D 2 O. Multiple replacement of hydrogen or
atoms by Cl, Br, and I occurs by reaction of the elemental
halogeninthedarkandcanbeforcedtofullreplacementof 1,6–Li 2 C 2 B 4 H 4 + 2Br 2 −→ 1,6–Br 2 B 4 H 4
all hydrogens. Elemental fluorine leads to extensive cage As in boron hydrides, bridge hydrogen atoms are the
degradation. Substitutions in which nitrogen, oxygen, or most acidic and can be removed with metal hydrides. Ad-
sulfur atoms are attached to the boron cage are known dition of acid to the carborane anion that is produced usu-
as are reactions in which one substitutent is replaced by ally reforms the original carborane, for example,
another. Examples are
2,3–C 2 B 4 H 8 + NaH −→ Na [2,3–C 2 B 4 H 7 ] + H 2
[H 3 O] 2 B 12 H 12 + CO −→ B 12 H 11 CO −
Na [2,3–C 2 B 4 H 7 ] + HCl −→ NaCl + 2,3–C 2 B 4 H 8
+ dicarbonylated species
[H 3 O] 2 B 12 H 12 + CH 3 CN −→ B 12 H 11 NCCH 3 B. Icosahedral Carboranes
[H 3 O] 2 B 12 H 12 + H 2 S −→ B 12 H 11 SH 2− The much explored chemistry of the icosahedral carbo-
ranes has concentrated on metallation followed by
(see Section IX)
metathesis reactions at the carbon atoms and electro-
Note that although the B 12 H 2− has all equivalent boron phylic substitution at the boron atoms. As with the nido
12
atoms, the B 10 H 2− ion does not. The two boron atoms in carboranes (above) the hydrogen atoms bonded to car-
10
apical positions that cap the square antiprism are each ad- bon are slightly acidic. Metallation reactions have been
jacent to only four other boron atoms whereas the other mainly performed using butyllithium and a large number
eightboronatomsinequatorialpositionsareeachadjacent of monofunctional substituents have been substituted onto
to five other boron atoms. In general, the equatorial boron the carbon atoms of ortho-, meta-, and para-carborane.