The active ingredient is borane, #"BH"_3#, but borane is a highly toxic gas.
Borane exists naturally as the dimer #"B"_2"H"_6# (diborane), but diborane mixes easily with air and forms explosive mixtures.
Also, it ignites spontaneously in moist air at room temperature.
In a solution in THF, borane exists as a loose Lewis acid-base complex. This allows boron to have an octet and makes the reagent more stable.
The solution is commercially available in a 1 mol/L concentration in volumes from 25 to 800 mL.
It is much more convenient to work with the solution than with a gas. Even so, the solution must be stored at 2 to 8 °C, and it must have a stabilizer added.
Borane forms a more stable and more soluble Lewis acid-base complex with dimethyl sulfide:
It is available in concentrations of 2, 5, and 10 mol/L and in volumes from 25 mL to 18 L.
That should make it a more convenient reagent than the BTHF complex.
There is only one problem: It has the highly disagreeable smell of rotten cabbage!