Fluoro anionic

Comment on heats of formation of fluoro-anions, and electron and fluoride-ion affinities of neutral fluorides, measured mass spectropho-tometrically (57,185,216,222) or derived from salt values obained by conventional calorimetry (32, 45, 46, 105) needs to be reserved until better agreement is reached between methods. However, from measurements on heats of formation of the predominantly ionic xenon fluoride adducts it has been possible to show the trend to increasing ionic-ity with pentafluoride partners Nb < Ta < Sb, which parallels the increasing Lewis acidity of these fluorides found by independent methods (44). 

The problems associated with the storage and handling of larger amounts of fluorine at high pressures can be avoided by the use of fluorine gas generators which are based on stable solids. Two practical approaches have been demonstrated,7 8 one of which is commercially available.9 Both approaches are based on high oxidation state complex fluoro anions of transition metals such as nickel, copper, or manganese. 

More recently, a quantitative scale for Lewis acidity based on fluoride ion affinities was calculated using ab initio calculations at the MP2/B2 level of theory.26 Due to its high basicity and small size, the fluoride ion reacts essentially with all Lewis acids thus the fluoride affinity (or reaction enthalpy) may be considered as a good measure for the strength of a Lewis acid. An abbreviated pF scale is given in Table 1.3. This scale was used recently by Christe and Dixon112 for estimating the stability of salts of complex fluoro anions and cations. The pF value represents the fluoride affinity in kcal mol 1 divided by 10. 

Salts of several bivalent fluoro anions have been prepared by this simple type of metathetical reaction. Thus (NF4)2TiF6 is formed from NF4SbF6 and Cs2TiF6 in AHF (48), and the hexafluoronickelate (39) and hexafluoromanganate (63) have been obtained similarly. 

This is the principal state for Pa, whose chemistry resembles that of Nbv and Tav. For Uv, Npv, Puv, and Amv, however, there is no particular resemblance to Nbv and Tav because of the importance of the An02 ions. Comparatively few solid Anv compounds, other than those of Pav and Puv are known the most notable are the Uv halides, the pentafluorides PaF5, NbF5, and UF5, fluoro anions, namely, AnF[Pg.1137]

An extremely important class of fluoro-anions will be presented in Sec. 11.3.5 where it will be shown that it is possible to generate fluoro-anions of transition metals in unusually high oxidation states, e.g. MnIVF, AgmF4 and NilvFj and to isolate the corresponding binary fluorides from HF solutions of these anions. 

The syntheses cited in this section have all involved attainment of high oxidation states in transition metal fluoro-anions, and thence in binary fluorides and in cationic species, by oxidation with F2 or other fluoro-oxidants in HF of metals or of compounds in lower oxidation states. A couple of examples are offered of syntheses of new fluoro-oxo-compounds involving fluorination of oxides already in high oxidation states with rare gas fluorides in HF. The ratio of F 0 in the ligands of these new compounds is greater than in the related compounds already reported. 

The nonchloroaluminate system has been intensively investigated, since fluoro-anions are also attractive candidates for RTILs [29, 60, 61]. The significant difference from the chloroaluminate system is its stability in air. The number of anions, which form RTILs with various cations, demonstrate an upward trend [40-45, 47, 62, 63]. 

Fortunately the chemical formulas can be rationalized on the basis of the size of the central atoms involved it is not necessary to memorize their formulas In general, second-period atoms are hmited to a maximum total coordination number (the total coordination number counts unshared electron pairs in the p-block) of fom third and fourth-period atoms can have maximum total coordination numbers of six fifth and sixth-period atoms can exceed a total coordination number of six. These observations explain the hydrolytic inertness (and persistence in the atmosphere) of compounds such as CF4 and SFe, which contain very strongly acidic cations they also explain why the formulas of fluoro anions vary (e.g. BF4 in period 2 AlFe in period 3 WFg in period 6). Evidently, because of the influence of Jt bonding to oxygen, central atoms in 0x0 anions fail to exhibit these coordination numbers, but instead settle for lower penultimate total coordination numbers 3 in the second period, for example, COs 4 in the third and fourth periods, for example, 8104 and 0004 6 in the fifth and sixth periods. 

The solubilities of salts in water (clearly of importance in aquatic, analytical, and geochemistry) can be fairly well predicted and explained using two principles later we shall return to the more familiar of these, the hard soft acid base (HSAB) principle (see Hard Soft Acids and Bases). To control this principle, we must first consider oifly salts of anions that are hard bases, that is, salts of 0x0 and fluoro anions, and oxides, hydroxides, and fluorides. The solubilities of salts of these anions can be fairly well predicted and explained on the basis of the acidity classification of the cation and the basicity classification of the anion, that is, on a principle of acid base strength. The numerous solubility rules taught in General Chemistry could be replaced with four solubility principles (Table 4), two of which are quite reliable and two of which are less reliable, for known reasons. 

The discovery of what appears to be a thermodynamic threshold governing the intercalation of graphite by fluoro anions, MF,, has required the evaluation of the thermodynamic stability of a number of such species. Since germanium tetrafluoride and fluorine are intercalated, in combination, by graphite to form both GeFj" and GeFs ", the first and second fluoride ion affinities of that molecule are each of interest. Evaluation of the fluoride ion affinity of boron trifluoride by Altshuller yielded a value of-71 kcal moF. This has been accepted by several authors as the basis for other fluoride ion affinities and electron affinities. Sharpe, however, has preferred a value of -91 kcal moF, based upon the data of Bills and Cotton. Although this latter value is in harmony with other fluoride ion affinities and electron affinities, its confirmation was clearly desirable to provide a firm basis for correction of affinities based upon the lower value. This paper describes the studies that have provided these fluoride ion affinities. 

This demonstrated that thermodynamic features dictated the extent of intercalation, at least for fluoro-anions. 

The Lewis superacids formed by the fluorides are a result of transfer of anions to form complex fluoro anions ... 

Anionic Species. Silicon forms only fluoro anions, normally, SiF2-, whose high stability constant accounts for the incomplete hydrolysis of SiF4 in water 2SiF4 + 2H20 -> Si02 + SiF -+2H++2HF... 

For the other elements, comparatively few solid compounds have been made. The halides are known only for Pa and U. Salts of fluoro anions such as MFg, MF,- and MF are known for Pa-Pu, although the Np and Pu compounds can be made only by solid-state reactions. Oxo chlorides, MOCI3, are known for Pa, U and Np. 

Al,N -dialkylacetainidinato, 622 dialkylamido, 609 dialkylcarbamato, 611 A, A -dialkyldithiocarbamato, 606 dialkyidithiophosphato, 639 difluorophosphinato, 639 P-diketonatooxo, 632 dithiocarbamatohalo, 607 fluoro anionic, 591 fluorooxo, 625 anionic, 626 halo... 

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