Fluoride affinities

Fluoride acceptors such as BF3, AsFs and SbFs readily form the anions [BF4], [AsFs] and [SbF ] respectively, and the F affinity for each acceptor can be determined using a thermochemical cycle such as that in scheme 6.23. 

---

The trifluorovinylxenonium salt (22, 27) and a series of c/.s-X-l,2-difluoroalkenylxenonium salts were successfully synthesized and isolated in 30-60% yields (22). It is noteworthy that the xenodeborylation reaction of highly acidic boranes, CF2=CFBF2 (fluoride affinity 80.3 kcal mol 1) and cis-CnF2n+iCF=CFBF2 (fluoride affinity 90.0 kcal mol 1 for n = 1) is accompanied by their partial conversion into perfluoroalkyltrifluoroborates, whereas in case of X = Cl the anion, [C1CF2CF2BF3] was not observed (eq 6). 

The next term in Equation (5) is the halide affinity of the metal halide, Ax (MtXJ. This is a very important term for the final value of AH4e and a term which, were it generally available, would facilitate worthwhile conclusions as to which metal halide would be the best anionogen. However, only two halide affinities for the metals of interest are known to us, viz. the fluoride affinities of BF3 and WF5 [26, 27]. In Scheme 3 the role of the halide affinity in the Born-Haber cycle for the BIE [Equation (iv)] is emphasised and an alternative route for the same energy contribution to AH4B is given. The individual BDE D(XBMt-X) is, like the halide affinity, known only for a very few examples of interest,... 

Silene 2, 1-fluorosilene 602 and 1,1-difluorosilene 603 have been examined by FT ion cyclotron resonance spectroscopy in order to estimate the jr-bond strength. Bracketing studies with various bases (B) and fluoride acceptors (A) gave values for the proton and fluoride affinity of the silenes284. The TT-bond energy was calculated by thermochemical cycles and found to increase with fluorine substitution. The data are given in Table 16. 

Silene Proton affinity (kcal mol-1) Fluoride affinity (kcal mol-1) 7r-Bond energy (kcal mol-1)... 

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. 

The BOF2 anion and BOF have been generated and detected in the gas phase by ion cyclotron resonance spectrometry [4], The BOF molecule has been established to have a fluoride affinity that is less than that of BF3. 

Since HF2 and other hydrofluoride anions must be small they can provide higher Coulomb energy than fluoroborate. In such cases, however, the fluoride affinity of A is far less (14) than in the BF3 case (5, 6). One must conclude that the favorable impact of the small anion size, as well as the Xe-F+ bond formation (see below), provides the driving force for XeF2 formation from Xe, F2 and HF via some XeF+F(HF)x intermediate. 

There are many unique and otherwise difficult reactions that are induced essentially by the strong interaction between fluorine and a special atom. Because of its hardness, fluorine interacts strongly with alkaline and alkaline earth metal ions, and in particular with neutral group III elements. Table 3.1 lists the strengths of chemical bonds between fluorine and other atoms, as a measure for the interaction between fluorine and other atoms [1], The theoretically calculated fluoride affinity also serves as a measure of interaction between a fluoride ion and an inorganic Lewis acid [2]. 

Boron is one of the hardest atoms and makes the strongest covalent bond (181 kcal / mol, Table 3.1) with fluorine and so the trivalent organic and inorganic boron compounds strongly interact both with fluoride ion and the organic fluorine in carbon-fluorine bonds. The calculated fluoride affinity proposed by Christe et al. [ 1 ] is also one of the measures for the interaction between fluoride ion and Lewis acids. 

The effects of fluorine on the relative stabilities of silicenium ions Me3 F Si+ (n=0-3) were studied by Murphy and Beauchamp36 using ICR. It was observed that the fluoride affinities, D[R+—F ], of the various fluoro- and methyl-substituted silicenium ions followed the order SiF3 >MeSiF2+>Me2SiF+>Me3Si +. Thus, a methyl substituent stabilizes the charge at Si+ relative to fluorine. Related studies on chlorosilanes are described in Ref. 36b. 

Tris(perfluoroalkyl)difluorophosphoranes (1) are very active compounds. They react readily with organic or inorganic fluorides [5-8] with formation of the corresponding salts with a FAP anion. The fluoride affinity of tris(perfluoro-alkyl)difluorophosphoranes is so high that by the reaction with HF in water the hydrolysis of phosphoranes practically completely suppressed and the corresponding tris(per luoroalkyl)tri luorophosphoric acid (2) is formed in nearly quantitative yield [Eq. (2)] [9],... 

Craciun R, Long RT, Dixon DA, Christe KO (2010) Electron affinities, fluoride affinities, and heats of formarion of the second row transition metal hexafluorides MFg (M = Mo, Tc, Ru, Rh, Pd, Ag). J Phys Chem A114 7571-7582... 

Craciun R, Picone D, Long RT et al (2010) Third row transition metal hexafluorides, extraordinary oxidizers, and Lewis acids electron affinities, fluoride affinities, and heats of formation ofWFe, ReFe, OsFe, IrFe, PtFe, and AuFe. Inorg Chem 49 1056-1070... 

More recently a quantitative scale for Lewis acidities has been reported, which is based on fluoride affinities. This pF scale (fluoride affinity values in kcal/mol divided by 10 Table 4) was determined using ab initio calculations (MP2/PDZ level of theory) (17). 

---