favor the T-shaped geometry of the hypervalent intermediates. For the selenium systems, atecomplexintermediates (20-Se, 26) were detected spectroscopically in THF- or THF/HMPA-containing solutions. A DNMR study showed that Li/Se exchange was substantially faster than exchange of the lithium reagents with the atecomplex. Therefore, these atecomplexes are not on the actual Li/Se exchange pathway.