The solid state structure of the low-melting compound Cl₂P–NMe₂ was determined by X-ray diffraction using an in situ grown single crystal. Two independent molecules in the asymmetric unit have very similar geometries with planar co-ordination at their nitrogen atoms and the C₂NP unit almost coinciding with the approximate molecular plane of symmetry. According to ab initio calculations up to the MP2/6-311G** level of theory the structure in the solid state corresponds to a transition state 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 C₁ symmetry with a gauche arrangement of the NMe₂ group. Both conformations of Cl₂P–NMe₂ are at variance to that determined by gas-phase electron diffraction in 1966, with a planar Me₂NP group being oriented perpendicular to the plane of symmetry. The calculations predict the latter structure not to be a stationary point on the potential hypersurface. The preference for the C_s structure of Cl₂P–NMe₂ in the crystal can be rationalised by the molecular dipole moments which are larger for the C_s structure than for the C₁ ground state. The results are discussed in comparison to the structure of F₂P–NMe₂ which was determined earlier in the gaseous and solid state, also with different geometries. New ab initio calculations for F₂P–NMe₂ are provided favouring C₁ symmetry, but showing the molecule to have a very small barrier to inversion of the nitrogen centre if any.