The reaction of (chloromethyl)trichlorosilane (7) with O-trimethylsilylated N,N-dimethylhydrazides yields initially under kinetic control, in nearly quantitative yields, a neutral hexacoordinate bis(NfSi) chelate, bis(N-(dimethylamino)acylimidato-N,O)chloro(chloromethyl)silicon(IV) (8a-c). Upon heating, the octahedral complexes 8 are quantitatively and irreversibly converted to the isomeric hexacoordinate chelates zwitterionic λ 6 -silicates 9a-c, (N-(dimethylamino). This molecular rearrangement is rather unusual, since it involves chelatering expansion from the common five-membered ring to a six-membered ring in the thermodynamically favored product 9. 8b and 9b were characterized by X-ray crystal analyses and in solution by their 1 H, 13 C, and 29 Si NMR spectra. The analogous compounds 8a,c and 9a,c were characterized by the respective analogy of their NMR spectra. Both 8b and 9b have slightly distorted octahedral geometries around the central silicon atom. Both series of chelates 8 and 9 undergo ligand-site exchange processes, observed by the coalescence of signals due to diastereotopic N-methyl and/or CH 2 groups. Each compound (of both series) undergoes two consecutive rate processes: in 8 this sequence of exchange processes is analogous to previously reported stereomutations through a bicapped-tetrahedral intermediate or transition state; in 9 the first (lower barrier) process is a nitrogen-silicon dissociationrecombination, and the high-barrier process is an inversion of configuration at the silicon center.We have recently utilized the reaction of O-(trimethylsilyl)-N,N-dimethylhydrazides (1) with polyhalosilanes as a convenient method for the synthesis of a variety of different penta-and hexacoordinate silicon complexes with NfSi coordination (eq 1). 1-9 1 functions as a precursor for five-membered NfSi chelate rings.On the other hand, when 1 was allowed to react with (chloromethyl)dimethylchlorosilane (4; eq 2), OfSi coordinated complexes with five-and six-membered chelate cycles (6 and 5, respectively) were obtained. 10-13 † Ben-Gurion University. Kalikhman, I.; Krivonos, S.; Stalke, D.; Kottke, T. J. Am. Chem. Soc. 1998, 120, 4209. (7) Mozzhukhin, A. O.; Antipin, M. Yu.; Struchkov, Yu. T.; Gostevskii, B. A.; Kalikhman, I. D.; Pestunovich, V. A.; Voronkov, M. G. Bannikova, O. B.; Petuchov, L. P.; Pestunovich, V. A.; Voronkov, M. G. Dokl. Akad. Nauk SSSR 1986, 287, 870. (11) Kalikhman, I. D.; Pestunovich, V. A.; Gostevskii, B. A.; Bannikova, O. B.; Voronkov, M. G. J. Organomet. Chem. 1988, 338, 169. (12) Macharashvili, A. A.; Shklover, V. E.; Struchkov, Yu. T.; Gostevskii, B. A.; Kalikhman, I. D.; Bannikova, O. B.; Voronkov, M. G.; Pestunovich, V. A.