Substituent contributions t o t h e 19F NMR exchange barriers of a series of silicates, [RR'SiF3]-, have been measured. Both electronic a n d steric effects control t h e barriers that are measured by variabletemperature experiments and analyzed by using complete bandshape analysis. Eight new silicates, as their K+-18-crown-6 salts, were prepared a n d studied (R, R'): (1) 4-methylphenyl, 4-methylphenyl; (2) 2-methylphenyl, 2-methylphenyl; (3) p-nitrophenyl, p-nitrophenyl; (4) a-naphthyl, a-naphthyl; (5) phenyl, 2,6-dimethylphenyl; (6) phenyl, tert-butyl; (7) 2,6-dimethylphenyl, 2,6-dimethylphenyl; (8) 2-methylphenyl, 2,6-dimethylphenyl.

Fluoride ion solubilized by 18-crown-6 has been reacted in aromatic solvents like toluene, o-xylene, and mesitylene with a variety of chloromethyl-and chloroalkyl-substituted silanes. Although both rearrangement and cleavage reactions have been demonstrated, this paper mainly deals with the effect of substituents on the rearrangement. In the case of some substituents like vinyl and aryl ones, secondary reactions occur. These are shown to result from the reaction of RCH2", where R = vinyl and aryl, and primary products like Me2(RCH2)SiF. Hammett relative reactivity studies are used to probe the details of both the primary and secondary reactions and demonstrate that for the primary reaction pentacoordination followed by rearrangement is controlled by substituents capable of supporting negative charge. To a greater extent this is also true of the secondary reaction substituent effects.