The molecular structures of tris(dimethylamino)-phosphane, -arsane and -stibane, E(NMe 2) 3, E  P, As or Sb and Me  CH 3, by gas electron diffraction and ab initio molecular orbital calculations

Abstract

The molecular structures of E(NMe 2) 3, E  P, As or Sb and Me  CH 3, have been determined by gas electron diffraction (GED) and ab initio molecular orbital calculations at the HF 6–31 G∗ level. The equilibrium structures have C s symmetry with two NMe 2 ligands oriented in such a manner that the direction of the electron lone pair on each N atom is orthogonal to the direction of the lone pair on the E atom, while the third ligand is oriented in such a manner that the lone pair on the N atom is antiparallel to the lone pair on E. The coordination of the antiparallel N atom is distinctly pyramidal (sum of the valence angles = 337° by GED) while the two orthogonal N atoms are nearly planar (sum of valence angles = 353° by GED). The bond distances from E to the antiparallel N atom is two to four pm longer than the bond distances to the orthogonal N atoms, and the valence angle >NEN spanned by the orthogonal N atoms is some 10° larger than the two angles spanned by the antiparallel and one orthogonal N atom. It is suggested that the equilibrium structures are stabilized by anomeric effects, i.e. delocalization of the lone pairs of the orthogonal N atoms into antibonding σ∗(E-N) orbitals.