ANALYSIS OF RIGID BODY AND NON-RIGID BODY MOTIONS IN X-RAY CRYSTALLOGRAPHY. F. Belaj, Institut für Anorganische Chemie, Universität Graz, Austria

Due to the high flexibility at the nitrogen atoms the [P(NPCl₃)₄]⁺ cation as a whole cannot be described as a rigid body not even with allowance for intramolecular torsions about bonds (J.D. Dunitz, D.N.J. White, Acta Cryst., 1973, A29, 93-94). The analysis of the thermal motion of this cation and of related species shows that the librations can best be described as almost independent motions of rigid tetrahedra, coupled by `ball joints'.

An accurate low-temperature structure determination of [P(NPCl₃)₄]⁺ ICl₂ was hindered by phase transition and the room-temperature structure, the cations lying on , could not be interpreted without some difficulties, too (F. Belaj, Acta Cryst., 1995, B51, 65-71). Recently the structure determination of [P(NPCl₃)₄]⁺PCl₆^- succeeded at 93K: The cations are located on mirror planes, thus three different NPCl₃ groups can be compared (see figure). The P-N bond lengths and P-N-P bond angles do not show such exceptionalities as in the ICl₂^- structure, but similar to the latter the PN₄ and NPCl₃ tetrahedra of the cation as well as the anion behave as rigid bodies with R_u<0.05 {R_u = [[[Sigma]]w([[Delta]]U)²/[[Sigma]]w(U)²]^1/2} , whereas for the cation as a whole an R_u value of 0.35 is obtained.