Tantalum chloride, also known as tantalum pentachloride, is the inorganic compound

Inorganic compound

Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...

 with the formula TaCl₅. This white powder is a starting material in tantalum

Tantalum

Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...

 chemistry. It hydrolyzes readily, releasing HCl. TaCl₅ is prepared by heating tantalum metal in chlorine

Chlorine

Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...

. Samples are often contaminated with tantalum(V) oxychloride (TaOCl₃), formed by hydrolysis.

Structure

TaCl₅ crystallizes in the monoclinic space group C2/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

Bridging ligand

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 restricted to small ligands such as pseudohalides or to ligands that are...

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

Physical properties

The solubility of tantalum pentachloride increases to a slightly for the following series of aromatic hydrocarbons: benzene< toluene< m-xylene< mesitylene, as 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 is also known to be a poor conductor of electricity, indicating little ionization. TaCl₅ is purified by sublimation

Sublimation (physics)

Sublimation is the process of transition of a substance from the solid phase to the gas phase without passing through an intermediate liquid phase...

 to give white needles.

Reactions

TaCl₅ is electrophillic and it behaves like a Friedel-Crafts type catalyst, similar to AlCl₃. It forms adduct

Adduct

An adduct is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components. The resultant is considered a distinct molecular species...

s with a variety of Lewis bases.

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 is a chloride donor and the latter serves as a ligand, binding through oxygen:

TaCl₅ + PCl₅ → [PCl₄⁺][TaCl₆]

TaCl₅ + OPCl₃ → [TaCl₅(OPCl₃)]

Tantalum pentachloride reacts with tertiary amines to give crystalline adducts.

TaCl₅ + 2 R₃N → [TaCl₅(NMe₃)

Chloride displacement reactions

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

TaCl₅ + 3 OPPh₃ → [TaOCl₃(OP(C₆H₅)₃]_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 an 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 + 5LiOMe → Ta(OMe)₄Cl + 4LiCl

Ammonolysis and amide-forming 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 dialkyamides:

TaCl₅ + 5LiNR₂ → Ta(NR₂)₅

Tantalum pentachloride is reduced by nitrogen heterocycles such as pyridine.

External links

• NIST Standard Reference Database