THE MOLECULAR-STRUCTURES OF TRIS(DIMETHYLAMINO)-PHOSPHANE, TRIS(DIMETHYLAMINO)-ARSANE AND TRIS(DIMETHYLAMINO)-STIBANE, E(NME2)(3), E = P, AS OR SB AND ME = CH3, BY GAS ELECTRON-DIFFRACTION AND AB-INITIO MOLECULAR-ORBITAL CALCULATIONS

The molecular structures of E(NMe2)(3), E = P, As or Sb and Me = CH3, have been determined by gas electron diffraction (GED) and ab initio m olecular orbital calculations at the HF/6-31G level. The equilibrium structures have Cs symmetry with two NMe2 ligands oriented in such a m anner that the direction of the electron lone pair on each N atom is o rthogonal to the direction of the lone pair on the E atom, while the t hird ligand is oriented in such a manner that the lone pair on the N a tom is antiparallel to the lone pair on E. The coordination of the ant iparallel N atom is distinctly pyramidal (sum of the valence angles = 337 degrees by GED) while the two orthogonal N atoms are nearly planar (sum of valence angles = 353 degrees by GED). The bond distances from E to the antiparallel N atom is two to four pm longer than the bond d istances to the orthogonal N atoms, and the valence angle drop NEN spa nned by the orthogonal N atoms is some 10 degrees larger than the two angles spanned by the antiparallel and one orthogonal N atom, It is su ggested that the equilibrium structures are stabilized by anomeric eff ects, i.e. delocalization of the lone pairs of the orthogonal N atoms into antibonding sigma(E-N) orbitals. (C) 1998 Elsevier Science B.V.