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The dimesitylneopentylsilene Mes 2 Si  CHCH 2 t Bu 1 is obtained in almost quantitative yield by reaction of tert-butyllithium with dimetisylvinylfluorosilane 4 ; 1 is certainly one of the most easily available stable silenes. In spite of its stability, 1 presents a high reactivity in the field of classical chemistry of organometallic alkenes such as addition or cycloaddition reactions and, in some cases, an original behaviour of ene-reagent (towards benzaldehyde) and both ene- and enophilic-reagent (towards acetophenone).

1,2-Dimesityltetrachlorodigermane 7 has been prepared by chlorination of 1,2-dimesityldigermane 6 with CCl 4 . From 7 , four new digermanes Mes(CHR 2 )Ge(X)Ge(X)(CHR 2 )Mes (CHR 2 = fluorenyl; X  H ( 4 ), Cl ( 5 ), OH ( 8 ) or F ( 9 ) have been synthesized. These sterically crowded derivatives are obtained as two diastereoisomers which have been isolated in the pure state by fractional crystallization. Reactions of various lithio compounds with the chloro or fluoro derivatives 5 or 9 lead to fluorene, following lithium-halogen exchange and then α-elimination and, in the case of t BuLi, a reduction of the germanium-halogen bond.

The dibisyl(flourenylidene)stannene 4 , obtained by dehydro-chlorination or -fluorination of the corresponding chloro- or fluoro-stannanes 5 and 6 by tert-butyllithium is an extremely air-sensitive compound. It has not been isolated, but identified by trapping reactions by lithio compound and water.

A new stable germaphosphene, the 2,2-dimesityl-1-(2,4,6-triipropylphenyl)germaphosphene ( 1 ), has been made. It reacts quantitatively with benzaldehyde and α-phenyl- N -tert-butylnitrone to afford, by [2+2]- and [2+3]-cycoloadditions, respectively, the correponding four- and five-membered heterocycles 10 and 13 . The conformations of these heterocycles are discussed. The reactions of 1 and 2,2-dimensityl-1-(2,4,6-tri-tert-butyphenyl) germaphosphene ( 2 ) with these reagents are compared.

The simple synthesis in nearly quantitative yield of dimesitylneopentylsilene 3 , the first stable silene in the neopentyl series, was performed using t -butyllithium and dimesitylvinyifiuorosilane. 3 was isolated by crystallization from pentane and characterized by 13 C (doubly bonded carbon at +110.4 ppm) and 29 Si (+77.6 ppm) NMR spectroscopy and analysis.

The formation of new π-bonded germanium intermediates with dicoordinated phosphorus atoms, germaphosphimines [R 2 2GePR′], has been observed in the exchange reaction between 2,5-disilaphospholanes and dialkyldihalogenogermanes. These transient species have been clearly characterized by insertion and ring expansion reactions on ethylene oxide and sulfide with formation of new heterocycles 2-germa-3-oxyphospholanes and 2-germa-3-thiophospholanes.

Aldehydes condense on the germaniumphosphorus bond of diethyl(triethylgermyl)phosphine with opening of the CO double bond and formation of phosphorus alkoxygermanes of the structure GeOCHP. The insertion derivatives of chloral and bromal are unstable and decompose via a β-elimination process. The addition of dialkylketones is far more difficult and only the ketones with a little hindered carbonyl group, such as cyclopentanone, or those activated by electron-attracting groups, such as hexafluoro-, hexachloroacetone, 1,1,3,3-tetrafluoro-1,3-dichloro- or 1,3,3-trifluoro-1,1,3-trichloropropanone lead to insertion derivatives which are not very stable [with the exception of (CF 3 ) 2 CO], which have been characterized by their transposition, decomposition, oxidation or hydrolysis products. α-Ethylenic aldehydes and ketones such as crotonaldehyde, cinnamaldehyde and 2-cyclopentene- 1 -one give 1,4-dipolar additions on their conjugated system, with formation of phosphorus enoxygermanes; hydrolysis of these derivatives leads to γ-phosphorus aldehydes or ketones. An addition mechanism involving an ionic cleavage of the germaniumphosphorus bond is considered. Ketenes insert in the germaniumphosphorus bond through an exclusive opening of the carbonyl group. Hydrolysis of the phosphorus enoxygermanes which are thus formed leads to “α-phosphoryl-ketones” which are also isolated by the action of germylphosphines on acid anhydrides.

Inversion at phosphorus in dimetalla-IVB-phospholanes, metalla-IVB-diphospholanes and in a diphosphanylgermylene has been studied by 1 H and 31 P dynamic NMR spectroscopy.