Lower symmetry than expected in some phosphines and phosphoranes

11 Lower symmetry than expected in some phosphines and phosphoranes 

If we were to predict the structure of a compound that has three equal substituents bound to a central phosphorus atom, we could be forgiven for assuming that it would have threefold symmetry. The structures of several compounds containing PN3 skeletons have been determined, and an early structure determination in 1969 of one of the simplest examples, P(NMe2)3, by gas electron diffraction was interpreted under this assumption. But the fit between the stmctural model and the observed molecular scattering intensities was not particularly convincing. 

Semi-empirical MO calculations on the smaller molecule P(NH2)3, later followed by P(NMe2)3, favored C3 structures as the global energy minima. However, these geometries were calculated to be only slightly lower in energy than those with symmetry. 

Would you expect to find stmctural distortions like those observed for P(NR2)3 in compounds such as RSi(NR2)3 and [S(NR2)3]+  

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