Pentaborane is a highly polar compound, with a dipole moment of 2.13 D.[9] It is soluble in hydrocarbons like benzene, and cyclohexane, and in greases including those used in lab equipment.

[10] Halogenation give the symmetrical derivatives B5H8X, which can be isomerised to place the halide on the base of the square pyramid.

The propellant mix that would produce the greatest specific impulse for a rocket motor is sometimes given as oxygen difluoride and pentaborane[citation needed].

During the early years of the space race and the missile gap, American rocket engineers thought they could more cheaply produce a rocket that would compete with the Soviets by using an existing first stage and putting an upper stage with an engine that produces thrust at a very high specific impulse atop it, so projects were begun to investigate this fuel.

Problems with this fuel include its toxicity and its characteristic of bursting into flame on contact with the air.

The US destroyed its last stockpiles of "Green Dragon" in 2000, long after the pentaborane had been discarded as unworkable.

The destruction procedure hydrolyzed the pentaborane with steam to yield hydrogen and a boric acid solution.

The long delay occurred in part because there are no industrial plants consuming pentaborane as a feedstock.

It can also readily form shock sensitive explosive compounds, and reacts violently with some fire suppressants, notably with halocarbons and water.

[15] The acute toxicity of pentaborane has caused it to be considered immediately dangerous to life and health, with a limit set at 1 ppm.