Structure and Motion of Tetrakis(trichlorophosphazeno)phosphonium Hexachlorophosphate, [P(NPCl₃)₄]⁺PCl₆^-, at 93 K

[P(NPCl₃)₄]⁺PCl^-₆, M_r = 880.02, monoclinic, P2₁/m, a = 7.720 (2), b = 21.235 (5), c = 8.248 (2) Å, = 91.12 (2)°, V = 1351.9 (6) Å³, Z = 2, D_x = 2.162 Mg m^-3, (Mo K) = 0.71069 Å, T = 93 K, R = 0.0497, S = 1.007 for 4536 unique observed reflections and 140 parameters. The cations and anions show site symmetries of C_s(m) and C_i(), respectively. The P-N bond lengths in the NPCl₃ groups are distinctly shorter [1.515 (4)-1.532 (4) Å] than those attached to the central P atom [1.595 (2)-1.611 (4) Å], the P-Cl bond lengths are 1.961 (1)-1.975 (1) Å in the cations and 2.132 (1)-2.142  (1) Å in the anions; the P-N-P angles lie in the range 132.5 (2)-142.9 (3)°. The structure is a derivative of the CsCl structure type showing interionic distances of 3.314 (1)-3.414 (1) Å, between the Cl atoms. The differences between the angles P-Cl_cation...Cl_anion [153.38 (5)-168.28 (5)°] and P-Cl_anion...Cl_cation [102.30 (3)-116.35 (4)°] are 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 = [w(U)²/w(U_obs)²]^1/2 = 0.024}, whereas the cation does not (R_u = 0.347). Allowing three intramolecular torsions the residual index R_u for the cation could be lowered to 0.184.

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