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302 Boron Hydrides

compounds are known, but they react to form a great vari- TABLE I Melting Points and Boiling Points of
ety of other compounds and ions. In addition, the prin- Stock’s Boron Hydrides
ciples of bonding and structure of the boron hydrides, Formula mp ( C) bp ( C)
◦
◦
which are somewhat unusual, apply to related compounds
in which one or more boron atoms have been replaced B 2 H 6 −165.5 −92.5
by atoms of different elements. The boron hydrides in- B 4 H 10 −120 16
clude gases, liquids, and relatively low-melting solids, B 5 H 9 −46.8 58.4
all of which have symmetrical structures. They have un- B 5 H 11 −123.3 65
pleasant odors, are toxic and thermally unstable, and react B 6 H 10 −65.1 108
with moisture and air. Many of the boron hydrides are B 10 H 14 99.5 ca. 231
pyrophoric (inflame spontaneously when exposed to air),
and any mixture of these compounds with oxidizing agents
earliest known boron hyrides are listed in Table I. Note
has the potential of explosive conflagration.
that at room temperature the first two compounds in this
We will begin this article by considering only the boron
table are gases, the next three are liquids, and the last is
hydrides, their formulas, structures, and a sufficient de-
a solid. New boron hydrides prepared since Stock’s work
scription of the theory of their bonding to understand why
range from B 6 H 12 to B 20 H 26 .
these molecules adopt their unique geometries. We will
The individual boron hydrides are named using a uni-
then expand our discussion to closely related compounds
versally employed system. The number of boron atoms
that share the same bonding principles and have analogous
in the compound is given by the prefixes di-, tri-, tetra-,
molecular shapes. The reactions of the boron hydrides and
penta-, hexa-, etc., preceding the word “borane.” The num-
their relatives will then be described in general terms. The
ber of hydrogen atoms in the compound follows as an Ara-
formation of interesting metal derivatives will be treated
bic number in parentheses. Thus B 5 H 11 is called pentab-
in a separate section. We will also describe possible im-
orane(11), pronounced pentaborane eleven.
portant applications of these boron compounds.
Most of the boron hydrides are difficult to work with
because they are unstable, and their availability is limited.
The two exceptions to this are pentaborane(9), B 5 H 9 , and
I. THE BORON HYDRIDES: THEIR
DISCOVERY AND FORMULAS decaborane(14), B 10 H 14 . Large quantities of B 5 H 9 remain
from a military jet fuel program in the 1950s (see Sec-
tion V). This liquid boron hydride can inflame if exposed
Evidence for the existence of boron hydrides appeared
to air. However, provided air is excluded B 5 H 9 is stable
late in the nineteenth century. However, means of handling
and a large number of its reactions have been explored and
compounds that are toxic and, especially, pyrophoric were
documented. The solid boron hydride B 10 H 14 does not in-
notavailable,andsoseriousexperimentionwasprecluded.
flame in the presence of air, and its chemistry has been
The first study giving actual chemical formulas was re-
studied to great advantage both in the vacuum line and on
ported in 1912 by the German chemist, Alfred Stock.
the benchtop. Existing supplies of B 10 H 14 are also largely
Stock had reacted magnesium metal with boron oxide to
the result of the 1950s jet fuel program.
form the refractory substance, magnesium boride,
6Mg + B 2 O 3 → 3MgO + Mg B 2
3
He then treated the magnesium boride with hydrochloric II. THE STRUCTURES OF
acidtoproduceamixtureofvolatileboronhydrides,which BORON HYDRIDES
we will give the general formula B x H y .
Theactualchemicalstructuresofboronhydridesremained
Mg B 2 + HCl → B x H y + 3MgCl 2 a mystery for decades. The obvious analogy of the formula
3
of diborane(6), B 2 H 6 , to ethane and of tetraborane(10),
Handling these compounds required that he invent an ap-
B 4 H 10 , to butane tempted speculation that the structures
paratus called the vacuum line in which chemical opera-
tions are performed in a sealed glass system that is under were also analogous. In fact, electron diffraction studies
a high vacuum. Subsequently, the vacuum line that Stock appeared to bear this out for B 2 H 6 , which was incorrectly
invented has been used for all manner of sensitive com- reported to have the ethane structure,
pounds and is a common and important piece of research
H H
laboratory apparatus.
Stock separated the above volatile boron hydride prod- H C C H
uct mixture into pure substances and then determined the
chemical formula and physical constants of each. These H H