STRUCTURE AND MOTION OF TETRAKIS(TRICHLOROPHOSPHAZENO)PHOSPHONIUM HEXACHLOROPHOSPHATE, [P(NPCL3)(4)]-, AT 93 K(PCL6)

[P(NPCl3)(4)]+PCl6-, M-r = 880.02, monoclinic, P2(1)/m, a = 7.720 (2), b = 21.235 (5), c = 8.248 (2) Angstrom, beta = 91.12 (2)degrees, V = 1351.9 (6) Angstrom(3), Z = 2, D-x = 2.162 Mg m(-3), lambda(Mo K alpha ) = 0.71069 Angstrom, T = 93 K, R = 0.0497, S = 1.007 for 4536 unique observed reflections and 140 parameters. The cations and anions show s ite symmetries of C-s(m) and C-i((1) over bar), respectively. The P-N bond lengths in the NPCl3 groups are distinctly shorter [1.515 (4)-1.5 32 (4) Angstrom] than those attached to the central P atom [1.595 (2)- 1.611 (4) Angstrom], the P-Cl bond lengths are 1.961 (1)-1.975 (1) Ang strom in the cations and 2.132 (1)-2.142 (1) Angstrom in the anions; t he P-N-P angles lie in the range 132.5 (2)-142.9 (3)degrees. The struc ture is a derivative of the CsCl structure type showing interionic dis tances of 3.314 (1)-3.414 (1) Angstrom between the Cl atoms. The diffe rences between the angles P-Cl-cation...Cl-anion [153.38 (5)-168.28 (5 )degrees] and P-Cl-anion...Cl-cation [102.30 (3)-116.35 (4)degrees] ar e very probably caused by the arrangement of the lone pairs at the Cl atoms. The thermal motion analysis showed that the anion behaves as a rigid body {R-u = [Sigma w(Delta U)(2)/Sigma w(U-obs)(2)](1/2) = 0.024 }, whereas the cation does not (R-u = 0.347). Allowing three intramole cular torsions the residual index R-u for the cation could be lowered to 0.184.