Spin Coupling through the N→Si Dative Bond in Penta- and Hexacoordinate Hydrido- and Fluorosilicon Complexes; Coupling through a Rapidly Dissociating−Recombining N→Si Bond

Abstract

Penta- and hexacoordinate silicon complexes with spin ½ nuclei ( 1 H and 1 9 F) directly attached to silicon have been prepared and used to study NMR spin-spin interactions across the N→Si dative bond. In both hydrido and fluoro complexes, two-bond coupling constants were found to drop sharply as the geminal bond angle deviated from 90°. Three-bond coupling constants showed a Karplus-type dependence upon the corresponding dihedral angles for both 1 H and 1 9 F complexes. Four-bond coupling constants 4 J( 1 9 F-Si-N-C- 1 H) across the dative bond were observed. The correlations can be used as tools for the interpolation of bond and dihedral angles from NMR data, in cases where crystal structures are unavailable. They also demonstrate that the N→Si dative bond in hypercoordinate silicon complexes behaves essentially in the manner expected from normal covalent bonds. Rapid dissociation and recombination of the N-Si dative bond was established in several pentacoordinate complexes; however, one-, two-, and three-bond coupling constants could nevertheless be measured across this rapidly dissociating bond.