Specific Complexation of R3Si by Nucleophilic Solvent Molecules

Specific complexation of R3Si+ by nucleophilic solvent molecules 

For a better understanding of the nature of solvated silylium ions in nucleophilic solvents the properties of R3Si(S)+ complexes were investigated for solvent prototypes S such as H2O, NH3 and HC1. [41] The possible formation of both tetra- and pentacoordinated complexes was considered. The two S molecules in pentacoordinated complexes were held at equal distances from the Si-atom in order to describe a situation in which the S molecules are part of a solvent shell 

IGLO/[7s6p2d/5s4pld/3slp]//HF/6-kcal/mol, Si shifts in ppm relative to 

and transfer of positive charge from R3Si+ to S in electrons derived from the Mulliken charge distribution. 

IGLO 29Si NMR shifts indicate that most of the complexes listed in Table 4 have only little silylium ion character. The shifts are in the range -130 to 30 ppm for complexes formed with ion 1, and in the range -45 to 180 ppm for complexes formed with ion 4. Hence, they are far from the ideal values of 321 (1, Table 1) and 381 ppm (4, Table 1) expected for free silylium ions. The complex formed with HC1 seems to contain some residual silylium ion character (53%) according to formula (3) of Section 4.1 although it is no longer justified to speak in this case of a free silylium ion in solution. 

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