Cationic species characterizing tetrahedrally coordinated

The synthetic efforts toward the isolation of silylium salts with an ideal trigonal-planar coordinated positively charged silicon atom in the cation created a series of stabilized sUyl cations in which either the interaction with the solvent, the counteranion, or intramolecular donor groups pacify the high electron demand of the silyl cation. This electron donation leads to cationic species 7 in which the silicon atom adopts a distorted tetrahedral coordination environment (Scheme 2) [9]. Examples for siliconium ions 8 in which the silicon atom has expanded its coordination number to 5 by addition of two solvent molecules are known but are less frequently observed. Intermolecular species 7 and 8 as well as intramolecular variants 9 and 10 for both modes of stabilization have been structurally characterized [9, 28, 29]. 

Incorporation of A1 as a T atom is readily checked by NMR spectroscopy. Figure 2 displays a typical NMR spectrum, showing how the possible coor-dinative states of A1 atoms may be distinguished on the basis of their NMR shift octahedral A1 atoms have a small chemical shift with respect to the reference hexa-aquo-Al cation, tetrahedral A1 atoms have a shift of ca. -60 ppm, and penta-coordinated species are characterized by an intermediate value. 

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