Tantalum(V) chloride

Tantalum(V) chloride, also known as tantalum pentachloride, is an inorganic compound with the formula TaCl₅. It takes the form of a white powder and is commonly used as a starting material in tantalum chemistry. It readily hydrolyzes to form tantalum(V) oxychloride (TaOCl₃) and eventually tantalum pentoxide (Ta₂O₅); this requires that it be synthesised and manipulated under anhydrous conditions, using air-free techniques.

Tantalum(V) chloride

Names
IUPAC names Tantalum(V) chloride Tantalum pentachloride
Identifiers
CAS Number	7721-01-9 TaCl5^N 17499-29-5 Ta2Cl10^N
3D model (JSmol)	Interactive image
ECHA InfoCard	100.028.869
EC Number	231-755-6
PubChem CID	24394
UNII	9WXV40ZI4M^N
CompTox Dashboard (EPA)	DTXSID7064780
InChI InChI=1S/5ClH.Ta/h5*1H;/q;;;;;+5/p-5
SMILES Cl[Ta](Cl)(Cl)(Cl)Cl
Properties
Chemical formula	TaCl5
Molar mass	358.213 g/mol
Appearance	white monoclinic crystals[1]
Density	3.68 g/cm^3
Melting point	216 °C (421 °F; 489 K)
Boiling point	239.4 °C (462.9 °F; 512.5 K) (decomposes)
Solubility in water	reacts
Solubility	chloroform, CCl4
Magnetic susceptibility (χ)	+140.0×10^−6 cm^3/mol
Structure
Crystal structure	Monoclinic, mS72
Space group	C2/m, No. 12
Thermochemistry
Std molar entropy (S^o298)	221.75 J K^−1 mol^−1
Std enthalpy of formation (ΔfH^⦵298)	-858.98 kJ/mol
Hazards
Flash point	Non-flammable
Lethal dose or concentration (LD, LC):
LD50 (median dose)	1900 mg/kg (oral, rat)
Related compounds
Other anions	Tantalum(V) fluoride Tantalum(V) bromide Tantalum(V) iodide
Other cations	Vanadium(IV) chloride Niobium(V) chloride
Related compounds	Tantalum(III) chloride, Tantalum(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

Structure

TaCl₅ crystallizes in the monoclinic space group C2/m.[2] The ten chlorine atoms define a pair of octahedra that share a common edge. The tantalum atoms occupy the centres of the octahedra and are joined by two chlorine bridging ligands. The dimeric structure is retained in non-complexing solvents and to a large extent in the molten state. In the vapour state, however, TaCl₅ is monomeric. This monomer adopts a trigonal bipyramidal structure, like that of PCl₅.[3]

Physical Properties

The solubility of tantalum pentachloride increases slightly for the following series of aromatic hydrocarbons:

benzene < toluene < m-xylene < mesitylene

This is reflected in the deepening of colour of the solutions from pale yellow to orange. Tantalum pentachloride is less soluble in cyclohexane and carbon tetrachloride than in the aromatic hydrocarbons. Such solutions of tantalum pentachloride are also known to be poor conductors of electricity, indicating little ionization. TaCl₅ is purified by sublimation to give white needles.

Synthesis

Tantalum pentachloride can be prepared by reacting powdered metallic tantalum with chlorine gas at between 170 and 250 °C. This reaction can also be performed using HCl at 400 °C.[4]

2 Ta + 5 Cl₂ → 2 TaCl₅

2 Ta + 10 HCl → 2 TaCl₅ + 5 H₂

It can also be prepared by a reaction between tantalum pentoxide and thionyl chloride at 240 °C

Ta₂O₅ + 5 SOCl₂ → 2 TaCl₅ + 5 SO₂

Tantalum pentachloride is commercially available, however samples can be contaminated with tantalum(V) oxychloride (TaOCl₃), formed by hydrolysis.

Reactions

TaCl₅ is electrophilic and it behaves like a Friedel–Crafts catalyst, similar to AlCl₃. It forms adducts with a variety of Lewis bases.[5]

Simple adducts

TaCl₅ forms stable complexes with ethers:

TaCl₅ + R₂O → TaCl₅(OR₂) (R = Me, Et)

TaCl₅ also reacts with phosphorus pentachloride and phosphorus oxychloride, the former as a chloride donor and the latter serves as a ligand, binding through the oxygen:

TaCl₅ + PCl₅ → [PCl⁺
₄][TaCl⁻
₆]

TaCl₅ + OPCl₃ → [TaCl₅(OPCl₃)]

Tantalum pentachloride reacts with tertiary amines to give crystalline adducts.

TaCl₅ + 2 R₃N → [TaCl₅(NR₃)]

Chloride displacement reactions

Tantalum pentachloride reacts at room temperature with an excess of triphenylphosphine oxide to give oxychlorides:

TaCl₅ + 3 OPPh₃ → [TaOCl₃(OPPh₃)]_x ...

The presumed initial formation of adducts between TaCl₅ and hydroxyl compounds such as alcohols, phenols and carboxylic acids is followed immediately by the elimination of hydrogen chloride and the formation of Ta–O bonds:

TaCl₅ + 3 HOEt → TaCl₂(OEt)₃ + 3 HCl

In the presence of ammonia as a HCl acceptor, all five chloride ligands are displaced with formation of Ta(OEt)₅. Similarly TaCl₅ reacts with lithium methoxide in anhydrous methanol to form related methoxy derivatives:

TaCl₅ + 4 LiOMe → Ta(OMe)₄Cl + 4 LiCl

Ammonolysis and alcoholysis and related reactions

Ammonia will displace most of the chloride ligands from TaCl₅ to give a cluster. Chloride is displaced more slowly by primary or secondary amines but the replacement of all five chloride centers by amido groups has been achieved by the use of lithium dialkylamides, as illustrated by the synthesis of pentakis(dimethylamido)tantalum:

TaCl₅ + 5 LiNMe₂ → Ta(NMe₂)₅

With alcohols, the pentachloride reacts to give alkoxides. As shown for the preparation of tantalum(V) ethoxide, such reactions are often conducted in the presence of base:

10 EtOH + Ta₂Cl₁₀ + 10 NH₃ → Ta₂(OEt)₁₀ + 10 NH₄Cl

Tantalum pentachloride is reduced by nitrogen heterocycles such as pyridine.

Reduction

Reduction of tantalum(V) chloride gives anionic and neutral clusters including [Ta₆Cl₁₈]⁴⁻ and [Ta₆Cl₁₄](H₂O)₄.[6]

Structure of edge-capped octahedral clusters such as Ta₆Cl₁₈²⁻.[7]

References

1. Lide, David R., ed. (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press. ISBN 0-8493-0487-3.
2. Rabe, Susanne; Müller, Ulrich (2000). "Crystal structure of tantalum pentachloride, (TaCl₅)₂". Z. Kristallogr. - New Cryst. Struct. 215 (1): 1–2. doi:10.1515/ncrs-2000-0102.
3. F. Fairbrother (1967). The Chemistry of Niobium and Tantalum. Elsevier.
4. Young, Ralph C.; Brubaker, Carl H. (1952). "Reaction of Tantalum with Hydrogen Chloride, Hydrogen Bromide and Tantalum Pentachloride; Action of Hydrogen on Tantalum Pentachloride". Journal of the American Chemical Society. 74 (19): 4967. doi:10.1021/ja01139a524.
5. F. A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry (4th ed.), Wiley, New York, 1980.
6. Duraisamy, Thirumalai; Hay, Daniel N. T.; Messerle, Louis (2014). "Octahedral Hexatantalum Halide Clusters". Inorganic Syntheses. 36: 1–8. doi:10.1002/9781118744994.ch1.
7. Thaxton, C. B.; Jacobson, R. A. (1971). "The Crystal Structure of H₂(Ta₆Cl₁₈)(H₂O)₆". Inorganic Chemistry. 10: 1460–1463. doi:10.1021/ic50101a029.CS1 maint: uses authors parameter (link)

Further reading

• Ozin, G. A.; Walton, R. A. (1970). "Vibrational spectra and structures of the 1:1 complexes of niobium and tantalum, pentachlorides and tantalum pentabromide with aceto-, perdeuterioaceto-, and propionitriles in the solid and solution states and a vibrational analysis of the species MX₅, NC·CY₃ (Y = H or D)". J. Chem. Soc. A: 2236–2239. doi:10.1039/j19700002236.
• Bullock, J. I.; Parrett, F. W.; Taylor, N. J. (1973). "Some metal halide–phosphorus halide–alkyl halide complexes. Part II. Reactions with niobium and tantalum pentachlorides and tungsten hexachloride". J. Chem. Soc., Dalton Trans. (5): 522–524. doi:10.1039/DT9730000522.
• Đorđević, C.; Katović, V. (1970). "Co-ordination complexes of niobium and tantalum. Part VIII. Complexes of niobium(IV), niobium(V), and tantalum(V) with mixed oxo, halogeno, alkoxy, and 2,2′-bipyridyl ligands". J. Chem. Soc. A: 3382–3386. doi:10.1039/j19700003382.
• Cowley, A.; Fairbrother, F.; Scott, N. (1958). "The halides of niobium (columbium) and tantalum. Part V. Diethyl ether complexes of the pentachlorides and pentabromides; the solubility of tantalum pentaiodide in ether". J. Chem. Soc.: 3133–3137. doi:10.1039/JR9580003133.

External links

• NIST Standard Reference Database