Protonic acids with complex anions

Combination of e.g. HF with BF3 or SbFs in a polar solvent should formally result in the formation of the corresponding protonic acid  

The stability of the complex anion depends on the nature of both Mt and X. The stability of complex anions decreases in the following order  

In general, this stability depends on geometric (crowding of halogen atoms) and electronic features (electronic structure of the central atom and its ability to acquire the negative charge). 

even relatively weak bases are protonated by the mixture HF — SbFs because of the low nucleophilicity and high stability of the formed SbFg anion l The stability is mostly due to the high affinity of SbFs towards fluoride anions, F . On the other hand, the proportion of protonation is much lower when less stable and more nucleophilic anions are used (e.g. AICI4). 

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The caged species may escape geminate recombination and produce various species that can initiate cationic polymerization. Solvent (RH) often participates in these reactions producing protonic acids. As shown in Eq. (44), protonic acids are also formed by reaction of radical cations with aryl radicals or by Friedel-Crafts arylation. Up to 70% of the protonic acid is formed upon photolysis of diaryliodonium salts [205]. In addition to initiation by protons, arenium cations and haloarene radical cations can react directly with monomer. The efficiency of these salts as cationic initiators depends strongly on the counterions. Those with complex anions such as hexafluoroantimonate, hexafluorophosphate, and triflate are the most efficient. 

If the nucleophilicity of the anion is decreased, then an increase of its stability proceeds the excessive olefine can compete with the anion as a donor for the carbenium ion, and therefore the formation of chain molecules can be induced. The increase of stability named above is made possible by specific interactions with the solvent as well as complex formations with a suitable acceptor 112). Especially suitable acceptors are Lewis acids. These acids have a double function during cationic polymerizations in an environment which is not entirely water-free. They react with the remaining water to build a complex acid, which due to its increased acidity can form the important first monomer cation by protonation of the monomer. The Lewis acids stabilize the strong nucleophilic anion OH by forming the complex anion (MtXn(OH))- so that the chain propagation dominates rather than the chain termination. 

As can be seen by Reactions 10.1-10.4, the state of the Stern layer depends on the chemistry of the solution it contacts. As pH decreases, the numbers of protonated sites (e.g., >(w)FeOH+) and sites complexed with bivalent anions (e.g., >(w)FeS04) increase. If protonated sites dominate, as is likely under acidic conditions, the surface has a net positive charge. 

The term demasking is used rather broadly to describe any process which reverses the process of masking. Control of the pH can be used to mask calcium ions in admixture with copper(II) whereby, in sufficiently acid solution, the latter is precipitated by oxine whereas the concentration of Ox- is reduced sufficiently by protonation to prevent the precipitation of calcium oxinate. Raising the pH will increase [Ox-] and both metals can now be precipitated. A more obvious example is the addition of fluoride ions to form the very stable complex anion SnF62- and so to mask tin(IV) against its precipitation as SnS2- Addition of boric acid will give BF4- and so demask the tin. 

The literature on metal complexes of carbohydrates through 1965 has been fully reviewed by Rendleman (I), and we shall therefore only discuss recent work. We shall not discuss the complexes formed with strong bases, such as calcium and barium oxide these are salts in which the sugar acts as a weak acid, losing one or several protons. Nor shall we discuss the complexes formed with anions of oxyacids—e.g., borate, stannate, periodate, etc. ions all these are compounds formed by covalent bonds in alkaline solution. We are concerned only with complexes formed with cations in neutral aqueous solution there is no evidence for the formation of complexes between sugars and simple anions in neutral aqueous solution. (For an example of complex formation between a sugar derivative and chloride ion in chloroform solution, see Reference 3.)... 

Table I lists the refined values of the log formation constants for Ni(II) and L-serine and L-threonine. Under the conditions of the study where the ligands may be present either as the zwitterion or the anion, both forms could interact with the Ni(II). The zwitterions, HL, are similar to the aliphatic hydroxy acids, and the anions of such compounds possess a carboxylate group through which unindentate complexes may be formed—e.g., Ni(II)-lactate, log K 2.216 (18). The values for the protonated Ni(II)-threonine and Ni(II)-serine complexes in this study were comparable (Table I).

The processes depend on the formation of the cyclohexadienyl anion intermediates in a favorable equilibrium (carbon nucleophiles from carbon acids with pKt > 22 or so), protonation (which can occur at low temperature with even weak acids, such as acetic acid) and hydrogen shifts in the proposed diene-chromium intermediates (25) and (26). Hydrogen shifts lead to an isomer (26), which allows elimination of HX and regeneration of an arene-chromium complex (27), now with the carbanion unit indirectly substituted for X (Scheme 9). 

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