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 Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that ...

chemistry. It readily

hydrolyzes Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysi ...

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 technique Air-free techniques refer to a range of manipulations in the chemistry laboratory for the handling of compounds that are air-sensitive. These techniques prevent the compounds from reacting with components of air, usually water and oxygen; less ...

Structure

TaCl₅ crystallizes in the

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...

space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it uncha ...

''C''2/''m''. 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 ligand In coordination chemistry, a bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually ...

s. 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

monomer In chemistry, a monomer ( ; '' mono-'', "one" + ''-mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization. Classification ...

ic. This monomer adopts a trigonal bipyramidal structure, like that of PCl₅.

Physical Properties

The solubility of tantalum pentachloride increases slightly for the following series of

aromatic hydrocarbon Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member of aromatic compounds is benzene. The word "aromatic" originates from the past groupin ...

s: :

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...

toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) a ...

< ''m''-xylene <

mesitylene Mesitylene or 1,3,5-trimethylbenzene is a derivative of benzene with three methyl substituents positioned symmetrically around the ring. The other two isomeric trimethylbenzenes are 1,2,4-trimethylbenzene (pseudocumene) and 1,2,3-trimethylbenze ...

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

with

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...

gas at between 170 and 250 °C. This reaction can also be performed using HCl at 400 °C. : 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 Thionyl chloride is an inorganic compound with the chemical formula . It is a moderately volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a chlorinating reagent, with approximately per year bein ...

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

adduct An adduct (from the Latin ''adductus'', "drawn toward" alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all co ...

s with a variety of Lewis bases.

Simple adducts

TaCl₅ forms stable complexes with

ethers In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom connected to two alkyl or aryl groups. They have the general formula , where R and R′ represent the alkyl or aryl groups. Ethers can again be ...

: :TaCl₅ + R₂O → TaCl₅(OR₂) (R = Me, Et) TaCl₅ also reacts with

phosphorus pentachloride Phosphorus pentachloride is the chemical compound with the formula PCl5. It is one of the most important phosphorus chlorides, others being PCl3 and POCl3. PCl5 finds use as a chlorinating reagent. It is a colourless, water-sensitive and moist ...

and

phosphorus oxychloride Phosphoryl chloride (commonly called phosphorus oxychloride) is a colourless liquid with the formula . It hydrolyses in moist air releasing phosphoric acid and fumes of hydrogen chloride. It is manufactured industrially on a large scale from pho ...

, the former as a chloride donor and the latter serves as a ligand, binding through the oxygen: :TaCl₅ + PCl₅ → [][] :TaCl₅ + OPCl₃ → [TaCl₅(OPCl₃)] Tantalum pentachloride reacts with tertiary amines to give crystalline adducts. :TaCl₅ + 2 R₃N → aCl₅(NR₃)

Chloride displacement reactions

Tantalum pentachloride reacts at room temperature with an excess of

triphenylphosphine oxide Triphenylphosphine oxide (often abbreviated TPPO) is the organophosphorus compound with the formula OP(C6H5)3, also written as Ph3PO or PPh3O (Ph = C6H5). This colourless crystalline compound is a common but potentially useful waste product in ...

to give oxychlorides: :TaCl₅ + 3 OPPh₃ → aOCl₃(OPPh₃)sub>''x'' ... The presumed initial formation of adducts between TaCl₅ and hydroxyl compounds such as alcohols,

phenols In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of one or more hydroxyl groups (— O H) bonded directly to an aromatic hydrocarbon group. The simplest is phenol, . Phenolic compounds are ...

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 Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

as a HCl acceptor, all five chloride ligands are displaced with formation of Ta(OEt)₅. Similarly TaCl₅ reacts with

lithium methoxide Lithium methoxide is a compound with formula LiCH3O. It is the lithium salt of methanol. Like other alkali metal alkoxides, lithium methoxide adopts a polymeric structure Its solubility in common polar aprotic solvents like THF is low; however, ...

in anhydrous methanol to form related methoxy derivatives: :TaCl₅ + 4 LiOMe → Ta(OMe)₄Cl + 4 LiCl

Ammonolysis and alcoholysis and related reactions

Ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

will displace most of the chloride ligands from TaCl₅ to give a cluster. Chloride is displaced more slowly by primary or

secondary amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent such ...

s 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 Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a ...

Reduction

Reduction of tantalum(V) chloride gives anionic and neutral clusters including a₆Cl₁₈sup>4− and a₆Cl₁₄H₂O)₄.

References

External links

NIST Standard Reference Database
{{DEFAULTSORT:Tantalum(V) Chloride Tantalum compounds Chlorides Metal halides