Hydrogen Fluoride-Tantalum Pentafluoride

Hydrogen Fluoride-Boron Trifluoride (Tetrafluoroboric Acid). 

Boron trifluoride ionizes in anhydrous HF [Eq. (2.29)]. The stoichiometric compound 

An ab initio study of BF3 + (HF) clusters115 has shown that up to n 3, only weakly bonded van de Waals associations are found but with n = 4-7, cyclic clusters were formed in which BF3 is hydrogen bonded to HF with 3 fluorine atoms. Microwave studies116 and IR investigations117 show that the intermolecular BFbond is nevertheless somewhat shorter than expected on the basis of pure van de Waals interactions. 

The hydrogen chloride-aluminum chloride system has been investigated by Brown and Pearsall118 under a variety of conditions, including temperatures as low as — 120°C by vapor pressure measurements. No evidence was found for a combination of the two components. 

---

Fluorides. Tantalum pentafluoride [7783-71-3] TaF, (mp = 96.8° C, bp = 229.5° C) is used in petrochemistry as an isomerization and alkalation catalyst. In addition, the fluoride can be utilized as a fluorination catalyst for the production of fluorinated hydrocarbons. The pentafluoride is produced by the direct fluorination of tantalum metal or by reacting anhydrous hydrogen fluoride with the corresponding pentoxide or oxychloride in the presence of a suitable dehydrating agent (71). The ability of TaF to act as a fluoride ion acceptor in anhydrous HF has been used in the preparation of salts of the AsH, H S, and PH ions (72). The oxyfluorides TaOF [20263-47-2] and Ta02F [13597-27-8] do not find any industrial appHcation. 

Perchloric acid (HCIO4 Ho —13.0), fluorosulfuric acid (HSO3F Ho — 15.1), and trifluoromethanesulfonic acid (CF3SO3H Ho —14.1) are considered to be superacids, as is truly anhydrous hydrogen fluoride. Complexing with Lewis acidic metal fluorides of higher valence, such as antimony, tantalum, or niobium pentafluoride, greatly enhances the acidity of all these acids. 

Fluorocomplexes may be prepared in various covalent fluorides as solvents, such as liquid hydrogen fluoride, bromine(III) fluoride, chlorine(III) fluoride, iodine(V) fluoride, arsenic(III) fluoride or even the pentafluorides of vanadium, niobium and tantalum. All of them seem to be associated in the liquid states due to fluorine-bonding and splitting of such bonds may lead to fluoride ion transfer processes between solvent molecules (autofluoridolysis) . 

---