Chemical Properties
yellow powder(s) or red crystals; ortho-rhomb, a=0.6127nm, b=1.2198 nm, c=1.855 nm; m.p. 265.2°C, b.p. 361.6°C; sensitive to moisture; can be formed by reacting Br2 and niobium at ~500°C [STR93] [KIR81]
Synthesis
(1) First, the air is completely removed from the reactor. Then,
the section of the tube containing the metal is heated by a coil wound directly on the reactor, or by a tubular electric furnace o.
If Nb powder is used, then bromide formation begins at 90°C; with
solid Nb, it starts at 195°C . The nascent bromide
sublimes onto cold finger /. When larger quantities are desired,
large-diameter receiver a is attachedtotheendof the reactor. The
extremely hygroscopic bromide should be removed from the
apparatus while the latter is in a dry box.
Nb + 5/2Br2 = NbBr5.
(2) In the method of Chaigneau, the mixture of pentoxide and AlBr3,
in proportions indicated by the above equation, is sealed under
vacuum into a Pyrex glass tube (before use, the AlBr3 is purified
by vacuum sublimation). The tube is heated for 24 hours at 200°C,
and allowed to cool; the small amounts of Br 3 formed in the process
and residual AlBr3 are vacuum-sublimed at 140°C. The pure
pentahalide is then separated from the Al 3O3 by vacuum sublimation
at 240°C, yielding large crystals.
3 Nb2O5 + 10AlBr3 = 6 NbBr5 + 5A12O
(3) Nb2O5 + 3 C + 6 Br2 = 2 NbBr5 + COBr2 + 2 CO2
