Neutral hexacoordinate silicon complexes 1 undergo an equilibrium dissociation to ionic siliconium chlorides 6 at low temperatures in polar solvents:  chloroform, dichloromethane, and fluorodichloromethane. The extent of dissociation increases with decreasing temperature, despite the formation of two ions from each molecule. The reaction enthalpies and entropies for the ionizations are negative, and their absolute values increase with increasing solvent polarity, indicating that solvation of the ions drives the dissociation process. The position of the equilibrium is readily controlled by variation of solvent polarity, temperature, replacement of the chloro ligand by better leaving groups (triflate, bromide, or tetrachloroaluminate), and variation of substituents (R) or ligands (X). The results are supported by crystal structures of the siliconium salts:  8a,c,d and 14d.

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 4 J(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.

Equilibrium between neutral hexacoordinate silicon complexes and ionic siliconium chlorides, which is highly temperature, solvent, counterion, ligand and substituent dependent, was observed by low temperature 29 Si NMR and confirmed by crystal analysis.

Binuclear hexacoordinate silicon chelates have been prepared and shown to have octahedral structure by X-ray crystallography. Their ionization in CD2Cl2 solution has been studied by 29Si NMR spectroscopy. Only one Si−Cl bond in 5a−c ionizes at low temperature to form the monosiliconium bis-chelates 11a−c. Use of a more acidic solvent, CHFCl2, facilitated the second ionization step to the disiliconium dichloride 12c. Replacement of the chloro ligands by better leaving groups (triflate, bromide, or iodide) caused complete ionization (7a−c, 8c, 9c) at room temperature. Crystal structure analyses of the binuclear siliconium triflates 7a,b show a square-pyramid geometry around the silicon atoms, with the ethylene bridge at the apex.

Neutral hexacoordinate silicon chelates with two chloro ligands (1) have been converted to neutral tris-chelates (6 and 7) by reaction with bis-(O,O-trimethylsilyl)catechol ( 4) and (Z)-Me 3 SiO(Ph)CdNNHSiMe 3 (5), respectively. Likewise, zwitterionic dichloro-hexacoordinate chelate (2a) was converted to the zwitterionic tris-chelate 8 with three different chelate rings, by reaction with 4. 2D-NOESY NMR spectra at various temperatures for 6a and 7a and for the chirally labeled 6b and 7c showed two consecutive ligand-site exchange processes, assigned to interchange of dimethylhydrazido-chelate rings via (O,O)-1,2-shift (6a, ∆G* ) 20.8 kcal mol -1 , 7a, 18.5 kcal mol -1 ) and dissociation-recombination of the N-Si bond. In 8, the NMR evidence shows that the lower process is N-Si dissociation-recombination, followed by enantiomerization at silicon via (O,O)-exchange (∆G* ) 16.1 and 20.9 kcal mol -1 , respectively). † Dedicated to Professor Mikhail Voronkov on the occasion of his 80th birthday.(1) For reviews on hypervalent silicon compounds see: (a) Chuit,