Niobium(V) chloride

Niobium(V) chloride

Sample of niobium(V) chloride
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Names
IUPAC names Niobium(V) chloride Niobium pentachloride
Identifiers
CAS Number	10026-12-7 NbCl5 17499-28-4 Nb2Cl10
EC Number	233-059-8
PubChem	24818
RTECS number	QU0350000
Properties
Chemical formula	NbCl5
Molar mass	270.17 g/mol
Appearance	yellow monoclinic crystals deliquescent
Density	2.75 g/cm3
Melting point	204.7 °C (400.5 °F; 477.8 K)
Boiling point	248.2 °C (478.8 °F; 521.3 K)
Solubility in water	decomposes
Solubility	soluble in alcohol, HCl, chloroform, CCl4
Thermochemistry
Std molar entropy (So298)	214.05 J K−1 mol−1
Std enthalpy of formation (ΔfHo298)	-797.47 kJ/mol
Vapor pressure	{{{value}}}
Related compounds
Other anions	Niobium(V) fluoride Niobium(V) bromide Niobium(V) iodide
Other cations	Vanadium(IV) chloride Tantalum(V) chloride
Related niobium chlorides	Niobium(III) chloride Niobium(IV) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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. NbCl₅ may be purified by sublimation.

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. NbBr₅, TaCl₅ and TaBr₅ are isostructural with NbCl₅, but NbI₅ and TaI₅ have different structures.

Preparation

File:Niobium pentachloride2.jpg

Niobium pentachloride liquid and vapor

Industrially, niobium pentachloride is obtained by direct chlorination of niobium metal at 300 to 350 °C:

2 Nb + 5 Cl₂ → 2 NbCl₅

In the laboratory, niobium pentachloride is often prepared from Nb₂O₅, the main problem being incomplete reaction to give the oxyhalides. The conversion can be effected with thionyl chloride: It also can be prepared by chlorination of niobium pentoxide in the presence of carbon at 300 °C. The products, however, contain small amounts of NbOCl₃.

Uses

Niobium(V) chloride is the main precursor to the alkoxides of niobium, which find niche uses in sol-gel processing. It is also the precursor to many other laboratory reagents.

In organic synthesis, NbCl₅ is a specialized Lewis acid in activating alkenes for the carbonyl-ene reaction and the Diels-Alder reaction. Niobium chloride can also generate N-acyliminium compounds from certain pyrrolidines which are substrates for nucleophiles such as allyltrimethylsilane, indole, or the silyl enol ether of benzophenone.

External links

• Safety information from ChemExper
• NIST Standard Reference Database