Silicon complexes with a new chelating donor group, the isopropylideneimino-acylimidato(N,O) moiety [OC(R)=NN=CMe2], have been prepared (9-12, 15), and their properties are compared with previously described silicon chelates (1, 2, and 16, respectively). The new ligand acts as a more powerful donor than the NNMe2 ligand, based on three criteria: (a) the new complexes form directly as pentacoordinate siliconium salts 9-12, i.e., the expected neutral hexacoordinate precursors ionize spontaneously; (b) comparison of crystallographic bond lengths with those of the NNMe2 complexes shows consistently shorter N-Si coordination bonds and longer Si-halogen bonds in equally substituted new relative to the previously studied complexes; (c) while in the previous series 1, the dihalo complexes 20-22, 25, resisted ionization at any temperature or solvent, the dibromo-isopropylideneimino complexes 19a and 19b ionize reversibly upon decrease of temperature in chloroform solution. Steric congestion forces the trans-dihalo configuration on the dichloro and bibromo complexes 18, 19. The ionization-resistant complexes 17, 18 and the partly ionized 19 form stable ionic siliconium salts when their counterions are replaced by I-, BPh4-, or by reaction with strong Lewis acids, AlCl3 and AlBr3. © 2003 Elsevier B.V. All rights reserved.