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Studies of a wide range of intramolecularly five-co-ordinated silicon compounds with chelating nitrogen donors have been made, with a view t o obtaining further information on the details of nucleophilic substitution reactions at silicon. The crystal structure of 2-(dimethylaminomethyl)phenyl-I -naphthylsilane has been determined b y single-crystal X-ray diffraction ( R = 0.074 for 564 observed reflections). Crystals are tetragonal, space group 14, with Z = 8 in a unit cell of dimensions a = 21.845(5) and c = 7.1 19(2) A. The central silicon atom shows essentially trigonalbipyramidal co-ordination, with the axial positions occupied by the naphthyl group and the donor nitrogen atom, and the t w o hydrogen atoms occupying equatorial positions. Comparison with other structures reported in the literature shows that this geometry, always with axial disposition of the donor atom, is generally adopted b y molecules of this type. Silicon-29, 19F, and 'H n.m.r. spectroscopic data for similar compounds with varying substituents on the silicon atom show that the same geometry is adopted by these intramolecularly co-ordinated species in solution.* This value was chosen as an average between the value previously reported for molecule (13) l 1 and other data from the literature (M.

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