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310 Boron Hydrides

the most stable boron hydrides, is unstable with respect to
its elements by 42.7 kJ/mol. The enthalpy of combustion
of pentaborane(9) calculated from standard enthalpies of
formation is −4512.55 kJ/mol or −71.63 kJ/g assuming
the reaction,
B 5 H 9 + 6O 2 −→ 2.5B 2 O 3 + 4.5H 2 O
In contrast, pentane has an enthalpy of combustion of only
−48.63 kJ/g.
Possible military applications of this high enthalpy of
FIGURE 12 The B 10 H 16 molecule. [From Shore, S. G., (1975).
combustion were considered seriously in the 1950s, and
In “Boron Hydride Chemistry” (E. L. Muetterties, ed.), p. 85, Aca-
several large plants for the manufacture of boron hydrides
demic Press, New York, Figure 3.5.]
were established. A “chemical bomber” was envisioned
with jet engines that would be propelled by ordinary hy-
drocarbon fuel except when making attacks at which time It is thought that the first step in the pyrolysis of B 2 H 6
the fuel supply would shift over to the higher energy boron is its dissociation to the unstable species BH 3 ,
hydride. Of possible boron hydrides, the two that were ac-
B 2 H 6 −→ 2BH 3 ,
tively considered involved the two most stable of these
compounds, B 5 H 9 and B 10 H 14 . Pentaborane(9) is a liquid The BH 3 then combines with the B 2 H 6 to produce an un-
within a reasonable temperature range but decaborane(14) stable B 3 hydride, which then undergoes a complex se-
is a solid. However, substitution of one of the terminal hy- ries of reactions with B 2 H 6 and other hydrides, stable and
drogen atoms on decaborane(14) by a methyl group pro- unstable, leading to the final mixture of boron hydride
duced a compound that did not pack in the crystal as well products.
as the much more symmetric decaborane(14) and thus was Two interesting and unusual boron hydrides are pre-
a liquid at a lower temperature range. pared when B 5 H 9 and B 10 H 14 are passed through a low
Various problems were to bedevil this project including pressure electric discharge. The product resulting from
the pyrophoric nature of the boron hydrides, the fact that passing B 5 H 9 through the discharge is B 10 H 16 with two
the B 2 O 3 combustion product was a solid, and that boron B 5 cages joined together by the borons at the apex of each
nitride, an extreme abrasive, was also produced. Both of square pyramid (B1) (Fig. 12). The electric discharge of
these products would be destructive to the internal work- B 10 H 14 produces B 20 H 16 , the only boron hydride having
ings of the jet engine. Furthermore, combustion of the fewer hydrogen atoms than boron atoms (Fig. 13). Appar-
boron hydrides appeared to be incomplete reducing the en- ently, there is insufficient space for the hydrogen atoms to
ergy of the reaction. Eventually the project was scrapped, bond to the four boron atoms in the concave belt around
but not before large amounts of B 5 H 9 and B 10 H 14 had the molecule.
been produced and two lives had been lost in pilot plant
accidents.
C. Reactions with Bases
The electrons provided by a base in its reaction with a
B. Polymerization and Electric
boron hydride allow ordinary single bonds to be produced
Discharge Reactions
at the expense of three-center bonds, and the energy of
Since they are relatively unstable compounds, boron hy- this bond formation drives the reaction. The simplest ex-
drides are subject to a variety of reactions involving input amples of this are the cleavage reactions of bases with
of energy, either thermal or electric discharge. Pyrolysis B 2 H 6 . Most bases except NH 3 cleave B 2 H 6 in what is
of lower boron hydrides such as B 2 H 6 or B 4 H 10 results in called a symmetrical manner,
polymerization producing a mixture of higher boron hy- L represents a base such as CH 3 NH 2 . Note that the start-
drides up through at least B 10 H 14 in sizable quantities plus ing diborane has a total of six bonds—four ordinary
hydrogen gas as a side product. The preparations of B 5 H 9
H
and B 10 H 14 are both based on pyrolysis. In addition to a
H H H
mixture of volatile boron hydrides, pyrolysis produces a + 2L 2 H
B B B L
quantity of a yellow, solid material of unknown composi-
H H H
tion that is capable of inflaming. H