Fluorosilicate anions

Common aqueous acids do not attack silica, except for hydrofluoric acid, which forms fluorosilicate anions, eg, SiF2-. The rate at which the various... 

Fluoromethylbenzoic acids, metallation, 9, 26-27 Fluoro(phenyl) complexes, with platinum(II), 8, 482 Fluorosilanes, elimination in fluorinated alkene activation, 1, 732 in fluorinated aromatic activation, 1, 731 and hydrodefluorination, 1, 748 Fluorosilicate anions, hypercoordinated anions, 3, 484 Fluorotoluenes, metallation, 9, 21 Fluorous alkylstannanes, preparation, 3, 820 Fluorous biphasic system, as green solvent, 12, 844 Fluorous ligands, with supercritical carbon dioxide, 1, 82 Fluorous media... 

Hypercoordinated fluorosilicate anions, preparation, 3, 484 Hypercoordinated silicon complexes... 

Photoreactions, pentacarbonyliron, 6, 3 Photoswitching, in hypercoordinated fluorosilicate anion... 

Careful purification of solutions of fluorosilane and fluorosilicate anions and a decrease in concentration and temperature result in a fine structure of F NMR spectra due to a non-equivalent arrangement of ligands (Table 16). For example, in the F NMR spectra of octahedral complexes of the type RSiFj" a doublet (equatorial atoms) and a quintuplet are observed whose mutual arrangement depends on the nature of the ligand In fluorine derivatives of penta- or hexa-coordinate silicon with slow exchange, the Hgip values (if they can be determined) are greater for the fluorine atoms with chemical shifts in low field. 

Six-coordinate silicon compounds result only when the silicon atom is bonded to highly electronegative atoms like fluorine, chlorine, oxygen, or nitrogen. A well-known example of this is the fluorosilicate anion SiF62, in which the fluorines are located in an octahedral array about silicon (322). The existence of (PhSiF5)2- in solution also has been confirmed by 19 F NMR spectroscopy (322). 

The low-temperature 19 F NMR spectra of fluorosilicate anions 220,221, and 222 show equivalent fluorine atoms satellites due to 29Si-,9F coupling are observed at -58°C. 

Berry pseudorotation as the mechanism of intramolecular exchange of ligands in pentacoordinate fluorosilicate anions represents a possible pathway. However, no definitive evidence has been presented for systems with mono-dentate ligands, and a pathway involving an intermediate hexacoordinated species can constitute an alternative mechanism. 

The axial Si-F bonds in the fluorosilicate anions listed in Table 21 are between 0.040 and 0.080 A longer than the respective equatorial Si-F bonds. An increase in the silicon coordination number is associated with an elongation in the Si-X bond distances, as exemplified by a comparison of SiF4311 and [SiF5] The equatorial Si-F bonds in the pentacoordinated anion are respectively 0.023 and 0.046 A longer than the Si-F bonds in the tetrahedral molecule SiF4. 

Discussion. The determination of this anion is of little practical importance. The methods available for its determination will, however, be outlined. Alkali fluorosilicates are decomposed by heating with sodium carbonate solution into a fluoride and silicic acid ... 

Reaction of allylic silanes with aldehydes and ketones can also be induced by fluoride ion, which is usually supplied by the THF-soluble salt tetrabutylammonium fluoride (TBAF). Fluoride adds at silicon to form a hypervalent anion with much enhanced nucleophilicity.73 An alternative reagent to TBAF is tetrabutylammonium triphenyldi-fluorosilicate.74... 

Several mechanisms for the peroxide oxidation of organosilanes to alcohols are compared. Without doubt, the reaction proceeds via anionic, pentacoordinate silicate species, but a profound difference is found between in vacuo and solvated reaction profiles, as expected. In the solvents investigated (CH2CI2 and MeOH), the most favorable mechanism is addition of peroxide anion to a fluorosilane used as starting material or formed in situ, followed by a concerted migration and dissociation of hydroxide anion. In the gas phase, and possibly in very nonpolar solvents, concerted addition-migration of H2O2 to a pentacoordinate fluorosilicate is also plausible. ... 

F3SiCH2CH2PMe2)2PdCl2 allowed the silicon atom to be penta-coordinated due to the Si<-Pd interaction . The F chemical shifts of anionic fluorosilicates correspond to the values for isolated ion only at inifinite dilution. The ion-pairs interaction between the hexafluorosilicates and various cations have been studied... 

Many other systems have subsequently been found to display anion conductivity. The electrical conductivity of lead fluoroborate and fluorogermanate glasses is similar to that of the lead fluorosilicate glasses. Calcium fluoroaluminate glasses also conduct by fluorine migration. Fluorophosphate glasses have also proven to be good anionic conductors. All of these systems are free of alkali ions. 

A variety of anionic catalysts, including fluorosilicates, bifluoride.carboxy-lates and other oxyanions are effective for the substitution reaction. Similar catalysts have been found useful for activating other silicon speciesl l. Bulk polymerizations using difunctional silyl ethers and vinyl ethers can be carried out without solvent, provided that very reactive catalysts are used. 

Activated by anionic catalysts (e.g., CsF, TAS fluorosilicates, carboxylates, fluoroalkoxides, fluorocarbanions, etc.), RfSiMes react readily with fluorinated acceptor molecules to give perfluoroalkylated products and trimethylfluorosilane. We have focused on acid fluorides and olefins as acceptors, although certain reactive aromatics and other carbonyl compounds function satisfactorily. 

LXIX. Steric hindrance in pentacoordinated fluorosilicates - synthesis and molecular-structure of the diphenyl-1-naphthyldifluorosilicate anion and the phenylmethyltrifluorosilicate anion, Inorg. Chem. 26 (5), 760-765 (1987). 

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