LOW-SYMMETRY IN P(NR(2))(3) SKELETONS AND RELATED FRAGMENTS - AN INHERENT PHENOMENON

A single-crystal X-ray diffraction study at 110 K and ab initio calcul ations up to the MP2/6-31G level of theory showed that the ground sta te of P(NMe(2))(3) has C-s symmetry, with two different coordination g eometries at the nitrogen atoms (pyramidal and planar). The calculated and experimentally determined geometries are in good agreement. Steri c strain could be ruled out as the predominant factor causing the devi ation from the expected C-3 symmetry, because ab initio calculations o n the model system P(NH2)(3) (up to MP2/6-311G*) also indicate C-s sy mmetry for the ground-state structure, whereas calculations on N(NH2)( 3) have predicted this molecule to have C-3 symmetry. The structure of the phosphorane H2C=P(NMe(2))(3) has been elucidated by X-ray diffrac tion in the solid state (110 K) and by electron diffraction in the gas phase augmented by restraints derived from ab initio calculations up to the MP2/6-31G level of theory. Solid-state and gas-phase structure s are in good agreement showing again the C-s arrangement to be the gr ound-state structure of the molecule. Ab initio calculations (MP2/6-31 1G*) indicate that H2C=P(NH2)(3) also has C-s symmetry. The reasons f or the preference for C-s rather than C-3 symmetry adopted by the P(NR (2))(3) units are discussed in terms of steric repulsion, lone pair, a nd other electronic interactions. Literature reports of structures of derivatives containing P(NMe(2))(3) units are discussed in the light o f the new results. Selected structural results include: P(NMe(2))(3) ( XRD, average values); P-N(1/2) 1.687, P-N(3) 1.731 Angstrom, N(1)-P-N( 2) 110.8, N(3)-P-N(1/2) 97.7 degrees, Sigma angle at N(1/2) 355.9, at N(3) 337.6 degrees; H2C=P(NMe(2))(3) (GED/XRD average without esd); P- C(1) 1.620(5)/1.655(6), P-N(1/2) 1.684(3)/1.668, PN(3) 1.718(6)/1.698( 4) Angstrom, N(1)-P-N(2) 115.2(13)/ 114.7(2), N(1/2)-P-N(3) 97.0(5)/99 .6, C(1)-P-N(1/2) 110.0(5)/110.0, C(1)-P-N(3), 127.1(8)/122.4(4)degree s, Sigma angle at N(1/2) 359.5(11)/353.4, at N(3) 332.9(13)/337.3(6)de grees.