PX3 species (X = Cl, Br, I) in imidazolium halide ionic liquids combine with the anion Z (Z = Cl, Br, I) of the solvent, forming [PX3Z] complex anions. These anions are found to have a sawhorse shape, with the lone pair of the phosphorus atom filling the third equatorial position of the pseudo trigonal bipyramid. Theoretical results show that this association remains incomplete due to the strong hydrogen bonding with the ionic liquid cations, which competes with the phosphorus trihalide for the interactions with the Z- anion. Temperature-dependent 31P NMR experiments indicated that the P-Z binding is weaker at higher temperature. Both theory and experiments evidence a dynamic exchange of the halide anions at the phosphorus atom, together with continuous switching of the ligands at the phosphorus atom between equatorial and axial positions. The detailed knowledge on the mechanism of the spontaneous exchange of halogen atoms at phosphorus trihalides suggests a way to design novel, highly conducting ionic liquid mixtures.