RECENT DEVELOPMENTS IN THE CHEMISTRY OF STABLE DOUBLY BONDED GERMANIUM COMPOUNDS

During the last 10 years, several compounds of the type Ge=X (X = C, G e, N, P, S) have been isolated as monomers. The stabilization of such derivatives, which are generally highly polymerizable, was achieved by using very bulky groups both on the germanium atom and on the heteroe lement X. Conjugation (particularly in a few germenes and germaimines) and intramolecular or intermolecular coordination with oxygen or nitr ogen, also contributes, in some cases, to the stabilization. The X-ray analyses of such compounds show a significant bond shortening of the double bond (8-10%) relative to the corresponding single bond and a pl anar or nearly planar germanium. These doubly bonded germanium derivat ives are usually thermally stable but must be handled in an inert atmo sphere because of their high sensitivity to oxygen and moisture; they are extremely reactive, much more than the corresponding carbon analog ues. Nearly quantitative additions on the double bond have been observ ed with electrophiles and nucleophiles, and various types of cycloaddi tions also occur. Except in one case, a germylene behavior has not bee n observed, proving that such compounds retain their structural integr ity in solution.