Influence of the transannular interaction in spiro-heterocycles containing Ge(IV) and Sn(IV). A comparative study

D(CH₂CH₂S)₂]M(XCH₂CH₂Y) 1-8 (M = Ge, Sn; D = O, S; X = Y = S, O and X = S, Y = O) spirocycles were synthesized to analyze the influence of the metal center replacement and the donor atom hardness on the strength of the transannular bond and the hypercoordination phenomena. The compounds were characterized by IR, Raman and NMR (¹H, ¹³C and ¹¹⁹Sn) spectroscopy, E.I. mass spectrometry and elemental analysis. The molecular and crystal structures of compounds 3, 4, 6-8 and Ge(SCH₂CH₂S)₂ (9) were obtained by X-ray diffraction analyses. They all exhibit five-coordinate central atoms due to transannular metal coordination (M?D) except 4, which displays a dimeric structure formed by the fusion of two five-membered rings resulting in a cyclic-distannoxane unit, {O(CH₂CH₂S)₂]Sn(SCH₂CH₂O)}₂. The relationship between the nature of the metal center and the differences found between the two germanium and tin series are discussed.