Niobium(V) chloride
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| Template:Longitem | NbCl5 |
| Molar mass | 270.17 g/mol |
| Appearance | yellow monoclinic crystals deliquescent |
| Density | 2.75 g/cm3 |
| Melting point | Template:Chembox CalcTemperatures |
| Boiling point | Template:Chembox CalcTemperatures |
| Solubility | HCl, chloroform, CCl4 |
| Template:Longitem | 214.05 J K−1 mol−1 |
| Template:Longitem | −797.47 kJ/mol |
| Flash point | Template:Chembox CalcTemperatures |
| Template:Longitem | Niobium(V) fluoride Niobium(V) bromide Niobium(V) iodide |
| Template:Longitem | Vanadium(IV) chloride Tantalum(V) chloride |
| Template:Longitem | Niobium(III) chloride Niobium(IV) chloride |
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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.[1]
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.[2] NbBr5, NbI5, TaCl5 TaBr5 and TaI5 are isostructural with NbCl5.
Preparation
Industrially, niobium pentachloride is obtained by direct chlorination of niobium metal at 300 to 350 °C:[3]
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In the laboratory, niobium pentachloride is often prepared from Nb2O5, the main challenge being incomplete reaction to give NbOCl3. The conversion can be effected with thionyl chloride:[4] It also can be prepared by chlorination of niobium pentoxide 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 compounds.
In organic synthesis, NbCl5 is a very 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.[5]
References
External links
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