THE MOLECULAR-STRUCTURE OF DICHLORO(DIMETHYLAMINO)PHOSPHINE

The solid state structure of the low-melting compound Cl2P-NMe2 was de termined by X-ray diffraction using an in situ grown single crystal. T wo independent molecules in the asymmetric unit have very similar geom etries with planar co-ordination at their nitrogen atoms and the C2NP unit almost coinciding with the approximate molecular plane of symmetr y. According to ab initio calculations up to the MP2/6-311G* level of theory the structure in the solid state corresponds to a transition s tate of inversion of the nitrogen pyramid, which is about 3.1 kJ mol(- 1) higher in energy than the ground state. The calculated ground state is of C-1 symmetry with a gauche arrangement of the NMe2 group. Both conformations of Cl2P-NMe2 are at variance to that determined by gas-p hase electron diffraction in 1966, with a planar Me2NP group being ori ented perpendicular to the plane of symmetry. The calculations predict the latter structure not to be a stationary point on the potential hy persurface. The preference for the C-s structure of Cl2P-NMe2 in the c rystal can be rationalised by the molecular dipole moments which are l arger for the C-s structure than for the C-1 ground state. The results are discussed in comparison to the structure of F2P-NMe2 which was de termined earlier in the gaseous and solid state, also with different g eometries. New ab initio calculations for F2P-NMe2 are provided favour ing C-1 symmetry, but showing the molecule to have a very small barrie r to inversion of the nitrogen centre if any.