Niobium(V) chloride, also known as niobium pentachloride, is a yellow crystalline solid. It hydrolyzes in air, and samples are often contaminated with small amounts of
NbOCl3. It is often used as a precursor to other compounds of
niobium
Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it has sim ...
. NbCl
5 may be purified by
sublimation
Sublimation or sublimate may refer to:
* ''Sublimation'' (album), by Canvas Solaris, 2004
* Sublimation (phase transition), directly from the solid to the gas phase
* Sublimation (psychology), a mature type of defense mechanism
* Sublimate of mer ...
.
Structure and properties
Niobium(V) chloride forms chloro-bridged dimers in the solid state (''see'' figure). Each niobium centre is six-coordinate, but the octahedral coordination is significantly distorted. The equatorial niobium–chlorine
bond lengths are 225 pm (terminal) and 256 pm (bridging), whilst the axial niobium-chlorine bonds are 229.2 pm and are deflected inwards to form an angle of 83.7° with the equatorial plane of the molecule. The Nb–Cl–Nb
angle at the bridge is 101.3°. The Nb–Nb distance is 398.8 pm, too long for any metal-metal interaction.
NbBr
5,
TaCl5 and TaBr
5 are isostructural with NbCl
5, but NbI
5 and TaI
5 have different structures.
Preparation
Industrially, niobium pentachloride is obtained by direct chlorination of niobium metal at 300 to 350 °C:
:2Nb + 5Cl
2 → 2NbCl
5
In the laboratory, niobium pentachloride is often prepared from Nb
2O
5, the main challenge being incomplete reaction to give
NbOCl3. The conversion can be effected with
thionyl chloride: It also can be prepared by chlorination of
niobium pentoxide
Niobium pentoxide is the inorganic compound with the formula Nb2 O5. A colorless, insoluble, and fairly unreactive solid, it is the most widespread precursor for other compounds and materials containing niobium. It is predominantly used in alloyi ...
in the presence of carbon at 300 °C.
Uses
Niobium(V) chloride is the main precursor to the alkoxides of niobium, which find uses in
sol-gel processing. It is also the precursor to many other Nb-containing reagents, including most
organoniobium compound Organoniobium chemistry is the chemistry of compounds containing niobium-carbon (Nb-C) bonds. Compared to the other group 5 transition metal organometallics, the chemistry of organoniobium compounds most closely resembles that of organotantalum com ...
s.
In
organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
, NbCl
5 is a very specialized
Lewis acid
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
in activating
alkenes for the
carbonyl-ene reaction
In organic chemistry, the ene reaction (also known as the Alder-ene reaction by its discoverer Kurt Alder in 1943) is a chemical reaction between an alkene with an allylic hydrogen (the ene) and a compound containing a multiple bond (the enophile) ...
and the
Diels-Alder reaction. Niobium chloride can also generate N-acyliminium compounds from certain
pyrrolidine
Pyrrolidine, also known as tetrahydropyrrole, is an organic compound with the molecular formula (CH2)4NH. It is a cyclic secondary amine, also classified as a saturated heterocycle. It is a colourless liquid that is miscible with water and most ...
s which are substrates for
nucleophile
In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
s such as allyltrimethylsilane,
indole, or the
silyl enol ether of
benzophenone.
References
External links
Safety information from ChemExperNIST Standard Reference Database
{{Chlorides
Niobium(V) compounds
Chlorides
Metal halides