Low‐Temperature Neutron Diffraction Study of the Silver Perchlorate–Benzene π Complex

The crystal structure of the AgClO 4 .C 6 H 6 π complex, earlier determined by X-ray diffraction at room temperature, has been redetermined at 18, 78 and 158K by neutron diffraction. Crystal data: orthorhombic, space group Cmcm, Z = 4, F(000)= 225.318 fm, T = 18, 78, 158K; D x = 2.591 (2), 2.570(1), 2. 523 (1) g cm -1 ; μ n = 0.166, 0.165, 0. 162 cm ―1 ; a = 7.913 (1), 7.973 (1), 8.100(1), 8.336(1)A (at 295 K); b = 7.837 (2), 7.857(1), 7.902(1), 7.996 (1) A (at 295 K); c = 11.798 (3), 11.777 (2), 11.739 (2), 11.638 (2) A (at 295 K); wR(F 2 ) = 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 + ...C 6 H 6 interactions are small, but significant. At 18 K the two C-C bonds complexed by Ag + are 1.405 (1) A in length; the other four are 1.398(1)A. The C-H bonds are equal in length at 1.087(2) (two) and 1.089(1)A (four). The H atoms nearest to Ag + are displaced 0.064(1)A from the C 6 plane, away from the silver. The shortest Ag + ...C distance of the complex is 2.565 (1) A. This value and bond lengths of the benzene molecule are invariant between 18 and 158K within 2 e.s.d.'s or less. The nonequivalent bond lengths of ClO 4 - , 1.451(1) (two) and 1.441(1) (two) at 18K, are foreshortened by -0.007 and -0.005A at 158K by effects of thermal motion. The O - Cl -O angles, 109.08(7), 109.98(2) and 107.83(8)° at 18K, are virtually unchanged by temperature. The Ag + ...ClO 4 - interactions occur at Ag + ...O distances of 2.785(1) and 2.612 (1) A (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 C 6 H 6 bond lengths at 18, 78 and 158K beyond their uncertainty limits. A harmonic potential rationalizes the motion of Ag + at these three temperatures.