The molecular and crystal structures of the tris(dimethylamino)phosphoranes (Me2N)(3)P-X (X = BH3, CH2, NH or O)

Single crystal X-ray diffraction studies and ab initio calculations up to t he MP2/6-31G* level of theory were carried out for the isoelectronic compou nds (Me2N)(3)PBH3, (Me2N)(3)PNH and (Me2N)(3)PO. The calculations predict a ll three compounds to adopt C-s symmetry in the ground state. The alternati ve geometries with local or total C-3 symmetry also correspond to local min ima on the potential hypersurface. In the solid state (Me2N)(3)PBH3 and (Me 2N)(3)PNH do not obey exact C-s symmetry, but are close to it, while (Me2N) (3)PO shows crystallographic mirror symmetry. The structural data are compa red with earlier studies on (Me2N)(3)PCH2 and the free phosphine P(NMe2)(3) , which both are also almost C-s symmetric. In the series (Me2N)(3)P-X (X = BH3, CH2, NH or O) the P-X bond lengths decrease, the P-N (pseudo) in plan e distances are 1.683(1), 1.698(4), 1.677(1) and 1.659(1) Angstrom, the (ps eudo) out of plane P-N distances are (av. = average) 1.654(av.), 1.668(av.) , 1.656(av.) and 1.649(1) Angstrom. The (pseudo) in plane angles X-P-N are 114.9(1), 122.4(4), 120.2(1) and 118.7(1)degrees, while the (pseudo) out of plane angles X-P-N are 112.3(av.), 110.0(av.), 110.4(av.) and 110.0(av.)de grees. In all structures the NMe2 groups in the (pseudo) plane of symmetry are steeply pyramidal at nitrogen, while the (pseudo) out of plane NMe2 gro ups are markedly flattened. The other structural parameters are also discus sed in comparison. Ab initio calculations (MP2/6-31G*) on the isoelectronic (Me2N)(3)PF+ predict this system to favour C-3 over C-s symmetry. In contr ast, the parent system (H2N)(3)PF+ is predicted to prefer C-s symmetry.