Tantalum(V) chloride

Tantalum(V) chloride, also known as tantalum pentachloride, is TaCl₅. This white powder is the main starting material in tantalum chemistry. It hydrolyzes readily, releasing HCl. TaCl₅ is prepared by heating tantalum metal in chlorine. Samples are often contaminated with tantalum(V) oxychloride, TaOCl₃, formed by hydrolysis or from traces of oxygen during the preparation. TaCl₅ is purified by sublimation to give white needles.

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Structure

TaCl₅ crystallizes in the monoclinic space group C_2/m. The unit cell contains six molecules of the dimer Ta₂Cl₁₀. In the dimer Ta₂Cl₁₀, 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-bridge bonds. 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.

Chemistry

TaCl₅ is strongly electrophillic and it behaves like a Friedel-Crafts type catalyst, similar to AlCl₃. TaCl₅ is known to form adducts with a variety of ligands.

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.

References

1. Cotton, F. Albert; Wilkinson, Geoffrey (1980). Advanced Inorganic Chemistry (4th Edn.). New York:Wiley. ISBN 0-471-02775-8. 
2. G.A. Ozin, R.A. Walton, J. Chem. Soc.(A). 1970, 2236-2239.
3. J. Bullock, F. Parrett, N. Taylor, J.C.S. Dalton 1973, 522-524.
4. C. Djordjevic, V. Katovic, J. Chem. Soc. (A). 1970, 3382-3386.
5. A. Cowley, F. Fairbrother, N. Scott, Halides of Niobium (Columbium) and Tantalum Part V. 1958, 3133-3136.
6. F. Fairbrother, The Chemistry of Niobium and Tantalum. Elsevier Publishing Company, 1967.
7. Rabe S, Müller U (2000). "Crystal structure of tantalum pentachloride". Zeitschrift für Kristallograpie-New Crystal Structures 216: 1-2.