Low-Temperature Neutron Diffraction Study of the Silver Perchlorate-Benzene [pi] Complex

The crystal structure of the AgClO₄.C₆H₆ complex, earlier determined by X-ray diffraction at room temperature, has been redetermined at 18, 78 and 158 K by neutron diffraction. Crystal data: orthorhombic, space group Cmcm, Z = 4, F(000) = 225.318 fm, T: 18, 17, 158 K; D_x = 2.591 (2), 2.570 (1), 2.523 (1) g cm^-1; _n = 0.166, 0.165, 0.162 cm^-1; a = 7.913 (1), 7.973 (1), 8.100 (1), 8.336 (1) Å (at 295 K); b = 7.837 (2), 7.857 (1), 7.902 (1), 7.996 (1) Å (at 295 K); c = 11.798 (3), 11.777 (2), 11.739 (2), 11.638 (2) Å (at 295 K); wR(F²) = 0.037, 0.035, 0.045, S = 1.18, 1.08, 1.10 for 782, 628, 800 reflections and 51 variable parameters. This study confirms the principal features reported in the X-ray investigation and reveals details of structure not observable at room temperature. Distortions of the benzene molecule from D_6h symmetry ascribed to Ag⁺...C₆H₆ interactions are small, but significant. At 18 K the two C-C bonds complexed by Ag⁺ are 1.405 (1) Å in length; the other four are 1.398 (1) Å. The C-H bonds are equal in length at 1.087 (2) (two) and 1.089 (1) Å (four). The H atoms nearest to Ag⁺ are displaced 0.064 (1) Å from the C₆ plane, away from the silver. The shortest Ag⁺...C distance of the complex is 2.565 (1) Å. This value and bond lengths of the benzene molecule are invariant between 18 and 158 K within 2 e.s.d.'s or less. The nonequivalent bond lengths of ClO^-₄, 1.451 (1) (two) and 1.441 (1) Å (two) at 18 K, are foreshortened by -0.007 and -0.005 Å at 158 K by effects of thermal motion. The O-Cl-O angles, 109.08 (7), 109.98 (2) and 107.83 (8)° at 18 K, are virtually unchanged by temperature. The Ag⁺...ClO^-₄ interactions occur at Ag⁺...O distances of 2.785 (1) and 2.612 (1) Å (18 K), where the shorter values involve ClO^-₄ acting as a bidentate group. Rigid-body and riding-motion models do not adequately account for the observed temperature dependence of bond lengths in ClO^-₄ nor provide significant corrections to the C₆H₆ bond lengths at 18, 78 and 158 K beyond their uncertainty limits. A harmonic potential rationalizes the motion of Ag⁺ at these three temperatures.

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