TETRAARYL-METHANE ANALOGS IN GROUP-14-4 - PH(4-N)M(P-TOL)(N) (N=0-4, M=SI,GE,SN,PB) - SYNTHESIS, STRUCTURAL AND SPECTROSCOPIC DATA, AND SEMIEMPIRICAL CALCULATIONS - MUTUAL INTERACTION OF TETRAHEDRAL SIGMA-ORBITALS (SYMMETRY AND ELECTRONEGATIVITY AND DELOCALIZED SIGMA-LUMOS (PI-LEWIS ACIDITY)

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 Ph(3)Sn(p-Tol), a survey of the 10 known structures and spectroscopic data (NMR, Mossbauer, IR, Raman) are given. A change of the symmetry of the formally tetrahedral MC(4) backbone arises if M = Si and Ge (elongation along one S-4 or C-3 axis) are altered to M = Sn and Pb (contraction along one S-4 axis). The order of delta(C-13-ipso) points to a decrease in the electronegativities along Pb much greater than Sn > Ge > Si. The Si-29, Sn-119 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 this equation is interpreted as an inductive contribution which changes its sign if M is changed from silicon to tin and lead. The quadratic part reflects the different population of a low-lying LUMO with charge given by the aromatic groups. This LUMO is slightly antibonding in the case of silicon and slightly bonding for tin and lead. The ''pi-acceptor'' properties of M explain the upfield NMR shifts Si-29/Sn-119/(207)pb of MAryl(4) compounds in comparison with MAlkyl(4).