Tantalum and Niobium Pentafluoride

Typical chain-type crystal structure is observed for compounds with the general formula MNbOF4. Table 30 presents cell parameters of tantalum and niobium pentafluorides and of other compounds with X Me = 5. 

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. 

The most universal method for the synthesis of tantalum and niobium fluoride compounds is based on direct interaction between their pentafluorides, TaF5 or NbFs, and fluorides of other metals. Some physical-chemical properties of these compounds are presented in Table 8 [71, 72]. 

Typical examples of compounds that form chain-type structures are simple pentafluorides of tantalum and niobium, TaFs and Nbfs, which were investigated by Edwards [199]. The cry stal structure of NbF< consists of rings made up of four NbF6 octahedrons that are linked by fluorine ligands that are each shared by two neighboring octahedrons, as shown in Pig. 29. The main interatomic distances (in A) are as follows Nb-Nb 5.80, 3.90 and 4.13 Nh-P,(Ki -. 1.75 and 1.78 Nb-Ftal% - 2.06 and 2.07 angle Nl>F[jf d,e-Nh - 182.5°. 

The major use of C1F3 is in the nuclear industry which converts unclean spent fuel reprocessing, uranium metal into gaseous uranium hexafluoride. Other applications are low temperature etchant for single crystalline silicon [63,64], It is also used as a fluorinating reagent and in the synthesis of GIF and conversion of metals to metal fluorides such as tantalum and niobium metals to tantalum pentafluoride and niobium pentafluoride, respectively. 

With increase in the acidity of the catalyst, methyl alcohol or dimethyl ether undergo condensation to saturated hydrocarbons or aromatics, with no olefin by-products. With tantalum or niobium pentafluoride based catalyst the studies at 300 °C resulted in conversion to gasoline range branched hydrocarbons and some aromatics (30% of the product) This composition is similar to that reported with H-ZSM-5 zeolite catalyst. 

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

Tantalum Pentafluoride, TaFs, is the only known fluoride of tantalum, and has been successfully isolated by methods that avoid hydrolysis (1) Tantalum and fluorine are brought into reaction exactly as in the preparation of niobium pentafluoride.1 (2) Tantalum penta-chloride is treated in the cold with dry hydrofluoric add the hydrochloric acid liberated and excess of hydrofluoric add are evaporated oft, and the resulting tantalum pentafluoride is purified by redistillation in a platinum crucible between 300° and 400° C.2 (3) The double barium tantalum fluoride, 3BaF2.2TaF5, is very strongly heated in a platinum tube, one end of which is kept cold. ... 

Fairbrother and Frith [38] found that a mixture of niobium and tantalum pentafluorides would polymerise isobutene in hexane solution at -78° only in the presence of a cocatalyst, such as trichloroacetic acid. 

The pentafluorides of niobium and tantalum are reported to be iso-stmctural with M0F5 (88). 

Double Fluorides of Tantalum Pentafluoride.—When solutions of tantalum pentoxide in hydrofluoric acid are treated with solutions of the fluorides of the alkali (and other) metals, double fluorides are obtained which possess the general formula R F.TaF5, where n usually varies between 1 and 3 in the most important series n=2. These double fluorides are much more stable than tantalum pentafluoride. They were among the first tantalum compounds to recdve examination,5 and still form an important class of tantalum compounds. A study of their isomorphism with the corresponding compounds of niobium... 

Niobium and tantalum pentafluorides are also effective catalysts for iodofluorination of TFE, while A1C13 and ZnCl2 show no activity. 

THE REACTIONS OF DIMETHYL SULFOXIDE WITH NIOBIUM AND TANTALUM PENTAFLUORIDES. 

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) . 

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