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.

Structure
TaCl5 crystallizes in the monoclinic space group C2/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.