Tetraaryl-methane analogues in group 14 - 4. Ph4-nM(p-Tol)n (n=0-4, M=Si, Ge, Sn, Pb). Synthesis, structuraland spectroscopic data, and semiempirical calculations: mutual interaction of tetrahedral σ-orbitals (symmetry and electronegativity) and delocalized σ*-lumos

Article abstract of DOI:10.1016/0277-5387(95)00069-5

The 20 compounds mentioned in the title have been synthesized by lithium(M=Si) or Grignard methods (M=Ge, Sn, Pb). The crystal structure of Ph3Sn(p-Tol), a survey of the 10 known structures and spectroscopic data (NMR, Moessbauer, IR, Raman) are given. A change of the symmetry of the formally tetrahedral MC4 backbone arises if M-Si and Ge (elongation along one S4 or C3 axis) are altered to M=Sn and Pb (contraction alongn one S4 axis). The order of δ((13)C-ipso) points to a decrease in the electronegativities along Pb>>Sn>Ge>Si. The (29)Si, (119)Sn and (207)Pb NMR chemical shifts exhibit a sagging along each series, which is described analytically in terms of a quadratic equation. The linear part of thisequation is interpreted as an inductive contribution which changes its sign if M is changed from silicon to tin and lead. The quadratic part re flects the different population of a low-lying LUMO with charge given bythe aromatic groups. This LUMO is slightly antibonding in the case of s ilicon and slightly bonding for tin and lead. The π-acceptor properties of M explain the upfield NMR shifts (29)Si/(119)Sn/(207)Pb of MAryl4 compounds in comparison with MAlkyl4.