Octahedral and Bicapped-Tetrahedral Silicon Configurations in the Solid State and Their Dynamic Coexistence in Solution

Hexacoordinate silicon bis chelates with the SiC2O2N2 ligand framework have been prepared, with
different nitrogen-donor ligands, NMe2 and NCMe2. Crystal diffraction analyses revealed that the former
type had a bicapped-tetrahedral geometry and the latter an octahedral molecular geometry in the solid
state. The two N→Si dative bonds in the isopropylideneimino-coordinated compound are sufficiently
strong to change silicon from its tetrahedral ground-state configuration to the higher energy octahedral
configuration. In contrast, the weaker NMe2 donor groups do not form strong enough bonds to cause this
change, and as a result silicon remains tetrahedral and forms only substantially longer and weaker N→Si
dative bonds (2.7−2.8 Å). Remote electron-releasing NMe2 substituents on the chelate rings render the
ligand NMe2 groups stronger donors, resulting in reversal of the solid-state geometry back to octahedral.
29Si, 1H, and 13C NMR spectra at various temperatures provide evidence for the coexistence in equilibrium
of the bicapped-tetrahedral and octahedral geometries in solution, with the octahedral to tetrahedral
population ratio increasing as the temperature is decreased.