The molecular structures of tris(dimethylamino)-phosphane, -arsane and -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 molecular orbital calculations at the HF/6-31G* level. The equilibrium structures have Cs symmetry with two NMe2 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 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 distances to the orthogonal N atoms, and the valence angle drop NEN spanned 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 suggested that the equilibrium structures are stabilized by anomeric effects, i.e. delocalization of the lone pairs of the orthogonal N atoms into antibonding sigma*(E-N) orbitals. (C) 1998 Elsevier Science B.V.