X-ray analysis of the first trigermaallene (3) and 1,3-digermasilaallene (4) revealed that the trimetallaallene skeletons are not linear but significantly bent, similarly to the corresponding trisilaallene 2. In contrast to 2, no dynamic disorder was observed for 3 and 4.

The reactions of stable cyclic dialkylgermylene 2 and dialkylstannylene 3 with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) radical (2 equiv) gave the corresponding 1:2 adducts 4 and 5, respectively, which were characterized by NMR, MS, and X-ray analyses. The kinetics of the stepwise addition of two TEMPO molecules to germylene 2 revealed that the initial addition of TEMPO to 2 was 1010 times slower than the second TEMPO addition to the resulting germyl radical. The origin of the rate difference was discussed on the basis of the qualitative perturbation theory. In contrast to the reactions of 2 and 3, the reaction of dialkylsilylene 1 with TEMPO gave an interesting 1,3-dioxadisiletane derivative.

The synthesis, X-ray and spectroscopic analysis, and unusual bonding and structure among heavy group-14 element congeners of allene (heavy allenes) [R(2)M=M'=MR(2); M = M' = Si (1a), M = Si, M' = Ge (1b), M = Ge, M' = Si (1c), and M = M' = Ge (1d)] are comparatively discussed. Using DFT calculations and PMO theory, the origin of the bent and fluxional skeleton of the heavy allenes is ascribed to the Jahn-Teller distortion associated with the effective pi-sigma* mixing (pi-sigma* distortion). This type of distortion is suggested to be a key concept for qualitative description of the unusual bonding of unsaturated compounds of heavy main-group elements.

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