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The silatranes basicity study results are summarized. A large amount of experimental data obtained 25-40 years ago is critically reviewed. It is shown that the reactivity and physicochemical properties of silatranes are determined by their basicity, which depends on the stereoelectronic structure.Silatranes substituted at the silicon atom XSi(OCH 2 CH 2 ) 3 N (XSa) represent a widely studied class of chelate compounds of pentacoordinated silicon. Investigators of silatranes have been attracted primarily by their unique crystal and stereoelectronic structure and also by their high biological activity. The systematic and comprehensive investigations of silatranes started by M. G. Voronkov and co-workers as far back as 1963 continue to this day [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Of particular practical interest here is the specific biological activity of silatranes [11][12][13][14]. Their unique reactivity is due both to the electron density distribution and the spatial structure.During study of the structure and properties of silatranes by numerous physicochemical methods [1-10] greatest attention has been paid to the axial fragment of their molecule and particularly to the length of the transannular N→Si bond. Insufficient attention has been paid to the equatorial fragment of the molecule. Here account was rarely taken of the fact that there is a redistribution of electron density during the transition of the silicon atom from the tetravalent state to the pentacoordinated state; in the silatrane molecule the positive charge of the central silicon atom and the negative charge of the surrounding three oxygen atoms and also of the axial substituent are increased [15][16][17].During studies of the silatranes basicity the attention of investigators was turned first of all to the basicity of the nitrogen atom, which was always considered a stronger nucleophile than the oxygen atom. Nevertheless, the ability of silatranes to form onium complexes with the participation of the nitrogen or oxygen atoms has been the subject of heated discussions in the literature. By this time in a letter to the editor, which initiated the story of silatranes (described at that time by the systematic name 5-aza-2,8,9-trioxa-1-silabicyclo[3.3.3]undecanes), Frye and co-workers unexpectedly announced that silatranes are not neutralized by perchloric acid in anhydrous acetic acid, but are only slowly titrated by it at 100°C. This led said authors to conclusion that the nitrogen atom does not have basic properties due to the presence of the transannular donoracceptor N→Si bond in the silatranes [18].

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