Fluorides, valency

Mendeleef based his original table on the valencies of the elements. Listed in Tables 1.6 and 1.7 are the highest valency fluorides, oxides and hydrides formed by the typical elements in Periods 3 and 4. 

From the tables it is clear that elements in Groups I-IV can display a valency equal to the group number. In Groups V-VII. however, a group valency equal to the group number (x) can be shown in the oxides and fluorides (except chlorine) but a lower valency (8 — x) is displayed in the hydrides. This lower valency (8 — x) is also found in compounds of the head elements of Groups V-VII. 

It is dissolved by bromine trifluoride, to form finally gold(III) fluoride, AuFj. This is a notable compound, for in it gold exhibits a simple valency of three, whereas in all other gold(III) compounds, gold is 4-coordinate, usually by complex formation (see below). 

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. 

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. 

CoF is used for the replacement of hydrogen with fluorine in halocarbons (5) for fluorination of xylylalkanes, used in vapor-phase soldering fluxes (6) formation of dibutyl decalins (7) fluorination of alkynes (8) synthesis of unsaturated or partially fluorinated compounds (9—11) and conversion of aromatic compounds to perfluorocycHc compounds (see Fluorine compounds, organic). CoF rarely causes polymerization of hydrocarbons. CoF is also used for the conversion of metal oxides to higher valency metal fluorides, eg, in the assay of uranium ore (12). It is also used in the manufacture of nitrogen fluoride, NF, from ammonia (13). 

Replacement of Hydrogen. Three methods of substitution of a hydrogen atom by fluorine are (/) reaction of a G—H bond with elemental fluorine (direct fluorination, (2) reaction of a G—H bond with a high valence state metal fluoride like Agp2 or GoF, and (J) electrochemical fluorination in which the reaction occurs at the anode of a cell containing a source of fluoride, usually HF. 

Reaction with a Metal Fluoride. A second technique for hydrogen substitution is the reaction of a higher valence metal fluoride with a hydrocarbon to form a fluorocarbon ... 

Fluorination of organic compounds using high valency metallic fluorides (14) may be represented as follows ... 

Electrolysis. Electrowinning of zirconium has long been considered as an alternative to the KroU process, and at one time zirconium was produced electrolyticaHy in a prototype production cell (70). Electrolysis of an aH-chloride molten-salt system is inefficient because of the stabiUty of lower chlorides in these melts. The presence of fluoride salts in the melt increases the stabiUty of in solution, decreasing the concentration of lower valence zirconium ions, and results in much higher current efficiencies. The chloride—electrolyte systems and electrolysis approaches are reviewed in References 71 and 72. The recovery of zirconium metal by electrolysis of aqueous solutions in not thermodynamically feasible, although efforts in this direction persist. 

Halogenation and dehalogenation are catalyzed by substances that exist in more than one valence state and are able to donate and accept halogens freely. Silver and copper hahdes are used for gas-phase reactions, and ferric chloride commonly for hquid phase. Hydrochlorination (the absoration of HCl) is promoted by BiCb or SbCl3 and hydrofluorination by sodium fluoride or chromia catalysts that form fluorides under reaction conditions. Mercuric chloride promotes addition of HCl to acetylene to make vinyl chloride. Oxychlori-nation in the Stauffer process for vinyl chloride from ethylene is catalyzed by CuCL with some KCl to retard its vaporization. 

For the past 20 years, four mam ways of replacement of hydrogen by fluorine have been used fluorination using elemental fluorine, electrochemical fluon nation, fluorination using high-valency metal fluorides, and selective electro philic fluorination... 

Some high-valency fluorides applied at moderate temperatures are capable of replacing individual hydrogens in aromatic rings Thus benzene affords fluoro benzene on treatment with silver difluoride [/] and with chlorine pentafluoride [2] (equations 1 and 2)... 

Changes in the charge of the central atom also strongly affect the metal-ligand bond length and the ionic-covalent share in fluoride complexes, which in turn impact the vibration spectra. Fig. 46 shows the dependence of asymmetric valence vibrations on the charge of the central atom. The spectral data for Mo, W, Zr, Hf fluoride compounds were taken from [71,115,137]. 

There is little new to be said about the bonding capacity of a lithium atom. With just one valence electron, it should form gaseous molecules LiH and LiF. Because of the vacant valence orbitals, these substances will be expected only at extremely high temperatures. These expectations are in accord with the facts, as shown in Table 16-1, which summarizes the formulas and the melting and boiling points of the stable fluorides of the second-row elements. In each case, the formula given in the table is the actual molecular formula of the species found in the gas phase. 

High-valency metal fluoride fluorination of pyridine [82JFC(21)171], quinoline [82JFC(21)413], and 2-methylfurans [91 JFC(51)179] has been reported. With 2-methylfuran a complex mixture of stereoisomers of partially fluorinated oxolans was obtained. These can be dehydrofluorinated to fluorooxolens and no furans have been observed. Conformation and structural group were found to influence the direction and readiness toward dehydrofluorination [91 JFC(52) 165]. 

The fluoride SF4 forms when a mixture of fluorine and nitrogen gases is passed over a film of sulfur at 275°C in the absence of oxygen and moisture. Write the Lewis structure of sulfur retrafluoride and give the number of electrons in the expanded valence shell. 

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