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

Penta- and hexacoordinate silicon complexes with spin 1/2 nuclei (1H and 19F) 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 1H and 19F complexes. Four-bond coupling constants 4J(19F−Si−N−C−1H) 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.