Tetraaryl-methane analogues in group 14 - V. Distortion of tetrahedral geometry in terms of through-space pi-pi and pi-sigma interactions and NMR sagging in terms of lambda-sigma* charge transfer

44 members of the compound series Ph4-nMRn (M = Si, Ge, Sn, Pb; R = o-, m-, p-Tol: n = 0-4) were synthesized (15 new compounds). The crystal structure s of Ph3Sn(o-Tol) and PhSn(o-Tol)(3) were determined and compared to 16 kno wn n structures. Subject to the distance d(M-C), an interplay between throu gh-space pi-pi repulsion and pi-sigma attraction leads to either elongated or compressed tetrahedral geometry. Si-29-, Sn-119- and Pb-307-NMR chemical shifts were determined in solution and in the solid state. Ge-73 chemical shifts were measured only in solution. An upfield or downfield sagging of t he chemical shifts along each series is rationalized in terms of a pi-sigma * charge transfer which is constrained by torsion of the aromatic groups. ( C) 1998 Elsevier Science Ltd. All rights reserved.