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Neutral hexacoordinate germanium complexes with hydrazido chelating ligands have been synthesized and characterized. Facile exchange of central element between silicon and germanium in these complexes is demonstrated, following given selectivity constraints.

O-Trimethylsilylated hydrazides with Schiff-base derivatives at the terminal nitrogen atoms-[RC(OSiMe 3 )dNNdCR 0 R 00 ] react with chloro(chloromethyl)dimethylsilane [ClCH 2 SiMe 2 Cl] in a regioselective manner, forming either five-membered or zwitterionic six-membered chelate complexes with pentacoordinate silicon. The type of product is determined by the size of the acidsubstituent R: bulky R groups (Ph, t-Bu) lead to exclusive formation of the six-membered product, whereas with the less bulky groups (Me, PhCH 2 ) only the five-membered product is obtained. Upon mild heating, the six-membered chelate complex transforms into its five-membered isomer, through a Wawzonek-type rearrangement. This facile transformation is remarkable in comparison to previously reported Wawzonek reactions that take place only at elevated temperatures. † Dedicated to Prof. Gerhard Roewer on the occasion of his 70th birthday.

Hexacoordinate dichelate silacyclobutane complexes have been synthesized from dichlorosilacyclobutane and O-trimethylsilylated hydrazides by transsilylation. Like previously reported hexacoordinate silicon complexes, they readily and quantitatively undergo ligand exchange with other silicon compounds (XSiCl3 and differently substituted O-trimethylsilylated hydrazides), evidence that ionic dissociation does not play a significant role in the exchange mechanism. Germanium tetrachloride causes central-element exchange and formation of analogous hexacoordinate germanium complexes. Likewise, silicon tetrachloride replaces germanium from its hexacoordinate complexes, obeying certain selectivity constraints. When silicon complexes have strongly electron-withdrawing chelate-ring substituents (CF3 or CH2CN), GeCl4 causes, in addition to central-element exchange, also oxidative opening of the four-membered ring and addition of two chlorine atoms. Both chelate exchange and central-element exchange are shown to be dominated by monodentate ligand priorities.

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