Niobium(V) chloride

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]

Niobium(V) chloride

Sample of niobium(V) chloride
Names
IUPAC names Niobium(V) chloride Niobium pentachloride
Identifiers
CAS Number	10026-12-7 NbCl5 Y[chemspider]
3D model (JSmol)	Interactive image dimer: Interactive image
ChemSpider	23203
ECHA InfoCard	100.030.042
EC Number	233-059-8
PubChem CID	24818
RTECS number	QU0350000
UNII	9S1BC7865F Y
CompTox Dashboard (EPA)	DTXSID7044348
InChI InChI=1S/5ClH.Nb/h5*1H;/q;;;;;+5/p-5 Key: YHBDIEWMOMLKOO-UHFFFAOYSA-I
SMILES Cl[Nb](Cl)(Cl)(Cl)Cl dimer: Cl[Nb-]1(Cl)(Cl)(Cl)[Cl+][Nb-]([Cl+]1)(Cl)(Cl)(Cl)Cl
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	HCl, chloroform, CCl4
Thermochemistry
Std molar entropy (S⦵298)	214.05 J K−1 mol−1
Std enthalpy of formation (ΔfH⦵298)	−797.47 kJ/mol
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H302, H312, H314, H332
Precautionary statements	P260, P261, P264, P270, P271, P280, P301+P312, P301+P330+P331, P302+P352, P303+P361+P353, P304+P312, P304+P340, P305+P351+P338, P310, P312, P321, P322, P330, P363, P405, P501
Flash point	Non-flammable
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). N verify (what is YN ?) Infobox references

Structure and properties

Ball-and-stick model of niobium pentachloride

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

Niobium pentachloride liquid and vapor.

Industrially, niobium pentachloride is obtained by direct chlorination of niobium metal at 300 to 350 °C:^[3]

2 Nb + 5 Cl₂ → 2 NbCl₅

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]