Fluorosilicates zwitterionic

The zwitterionic A5S7-fluorosilicates 4-22 were isolated as crystalline solids. Compounds 4-8, 13, and 17-22 were structurally characterized in the solid state by single-crystal X-ray diffraction. In contrast to the achiral zwitterions 4-16, the zwitterions 17-22 are chiral, the respective crystals consisting of pairs of enantiomers [(A)- and (C)-enantiomers]. In all cases, the /-coordination polyhedron was found to be a somewhat distorted trigonal bipyramid, with fluorine atoms in the two axial sites. This is illustrated for 6 and 19 in Fig. 1. Selected geometric parameters for compounds 4-8, 13, and 17-22 are listed in Table I. As can be seen from these data, the axial Si-F distances [1.647(2)-1.743(1) A] are significantly longer than the equatorial ones [1.589(2)-1.638(1) A]. The Si-Cl distances amount... 

An interesting reaction found in zwitterionic fluorosilicates is their transformation from acyclic to monocyclic complexes (equation 5), with oxygens replacing fluorine ligands30a. 

FIGURE 5. X-ray crystallographic structure of 9 (zwitterionic fluorosilicate). Reproduced from Reference 31a by permission of VCH Verlagsgesellschaft... 

A new direction in the chemistry of hypervalent silicon compounds has recently been developed by Tacke and coworkers the chemistry of zwitterionic organosUicates ". In these compounds the silicon is formally negative, with a cationic nitrogen attached to it by an alkyl chain. One of the groups of products of this general class is the pentacooidinate zwitterionic fluorosilicates. ... 

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