Tantalum(V) chloride, also known as tantalum pentachloride, is an inorganic compound with the formula TaCl5. It takes the form of a white powder, which is highly sensitive to moisture. It is synthesised and manipulated under anhydrous conditions, using air-free techniques.
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 ligands. The dimeric structure Ta2Cl10 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.
Synthesis Tantalum pentachloride can be prepared by treating powdered metallic tantalum with chlorine 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 at 240 °C.
Ta2O5 + 5 SOCl2 → 2 TaCl5 + 5 SO2 Tantalum pentachloride is commercially available, however samples can be contaminated with tantalum(V) oxytrichloride (TaOCl3), formed by hydrolysis. Samples contaminated with niobium pentachloride appear yellow.
Reactions TaCl5 is electrophilic. It forms adducts with a variety of Lewis bases. It serves as a Lewis acid in Friedel-Crafts reactions.
Simple adducts TaCl5 forms stable complexes with ethers:.
TaCl5 + R2O → TaCl5(OR2) (R = Me, Et) TaCl5 also reacts with phosphorus pentachloride and phosphoryl chloride, the former as a chloride donor and the latter serves as a ligand, binding through the oxygen:
TaCl5 + PCl5 → [PCl4]+[TaCl6]− TaCl5 + OPCl3 → [TaCl5(OPCl3)] Tantalum pentachloride reacts with tertiary amines to give 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 ...
Hydrolysis, alcoholysis, ammonolysis and related reactions Anhydrous TaCl5 (Ta2Cl10) rapidly (but not violently) hydrolyses in water at room temperature. Tantalum pentachloride reacts with sodium ethoxide to give the pentaethoxide, which also exists as a dimer:
5 NaOC2H5 + TaCl5 → Ta(OC2H5)5 + NaCl A variety of related alkoxides and mixed chloro-alkoxides are known, e.g. Ta(OCH3)5 and Ta(OCH2CH3)3Cl2. Ammonium chloride gives the nitrido complex [TaCl5N]3−.
Reduction Reduction of tantalum(V) chloride gives anionic and neutral clusters including [Ta6Cl18]4− and [Ta6Cl14](H2O)4.
References
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. 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.
External links
NIST Standard Reference Database
