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Boron Hydrides 311

charge. Reactions of certain boron hydrides with water can
lead to the abstaction of boron atoms and the production
of a smaller boron hydride and hydrogen gas,

B 6 H 12 + 6H 2 O −→ 2B(OH) 3 + B 4 H 10 + H 2

D. Reactions with Reducing Agents
Because of their deficiency in the electrons necessary for
the formation of all two-center bonds, reactions of boron
hydrides with reducing agents would be expected to pro-
ceed easily. Reactive metals will contribute electrons to
boron hydrides to produce boron hydride anions, for ex-
FIGURE 13 The B 20 H 16 molecule. All boron atoms except those
shaded in gray bear terminal hydrogen atoms. [From Muetterties, ample,
E. L., ed. (1975). In “Boron Hydride Chemistry,” p. 9, Academic B 2 H 6 + 2Na(amalgam) −→ 2Na + B 2 H 2−
+
Press, New York, Figure 1.5.] 6
B 5 H 9 + 2Na(amalgam) −→ 2Na + B 5 H 2−
+
9
two-center bonds and two three-center bonds—whereas
B 10 H 14 + 2Na −→ 2Na + B 10 H 2−
+
14
the two molecules of product have a total of eight bonds,
all two-centered.
Ammonia cleaves B 2 H 6 in an unsymmetrical manner
E. Reactions with Electrophylic Reagents
giving an ionic product with both NH 3 groups bonded to
the same boron atom, Electrophylic substitution is very familiar in substitution
reactions of aromatic organic compounds such as the
H H H
bromination of benzene using aluminum bromide as a
B B + 2 NH 3
H H H catalyst. Such reactions can also be performed on some
of the boron hydrides. However, the conditions of elec-
+ −
NH 3 H trophylic substitution are rigorous enough that the less
stable of the boron hydrides would decompose under the
H B NH 3 + H B H reaction conditions and fail to produce significant isolable
products. These reactions have, in fact, been explored in
H H detail for the two more stable hydrides, B 5 H 9 and B 10 H 14 .
Analogous cleavage reactions have been observed with Since the attacking species in an electrophilic substitution
B 4 H 10 and B 5 H 11 . Such basic cleavage reactions are only is positively charged or is at the positive end of a dipole,
observed to occur in boron hydrides in which a boron knowledge of the charge distribution on the boron hydride
atom is bonded to the rest of the molecule by two boron- should predict accurately the location of substitution, and
hydrogen-boron, three-center bonds. this is, in fact, the case.
Boron hydrides form a considerable number of Lewis A very simple method of calculating the charge distri-
base adducts, i.e., compounds in which the Lewis base butions on boron hydrides can be performed by making the
has become bonded to the boron hydride with the boron
framework intact. Such compounds have been observed
for B 5 H 9 ,B 6 H 10 ,B 8 H 12 , and B 10 H 14 . The reaction of
B 10 H 14 with a Lewis base to produce an intermediate nec-
essary for the preparation of ortho-carborane has been
discussed above. The structure of this adduct is given in
Fig. 14.
The bridge hydrogens of a boron hydride are the most
acidic, and, in fact, B 10 H 14 is a strong acid. Titration of
B 10 H 14 can be performed with a strong base.
FIGURE 14 The general structure of B 10 H 12 L 2 compounds, the
−
−
B 10 H 14 + OH −→ B 10 H + H 2 O Lewis base adducts of B 10 H 14 . L represents a Lewis base such as
13
CH 3 CN or (CH 3 ) 2 S. [From Shore, S. G. (1975). In “Boron Hydride
Terminal hydrogen atoms in boron hydrides are hy- Chemistry” (E. L. Muetterties, ed.), p.137, Academic Press, New
dridic, that is, behave as though they have a partial negative York, Figure 3.40.]