Weakly Coordinating Anions:  Crystallographic and NQR Studies of Halogen−Metal Bonding in Silver, Thallium, Sodium, and Potassium Halomethanesulfonates

<sup>35</sup>Cl, <sup>79,81</sup>Br, and <sup>127</sup>I NQR (nuclear quadrupole resonance) spectroscopy in conjunction with X-ray crystallography is potentially one of the best ways of characterizing secondary bonding of metal cations such as Ag<sup>+</sup> to halogen donor atoms on the surfaces of very weakly coordinating anions. We have determined the X-ray crystal structure of Ag(O<sub>3</sub>SCH<sub>2</sub>Cl) (<i>a</i> = 13.241(3) Å; <i>b</i> = 7.544(2) Å; <i>c</i> = 4.925(2) Å; orthorhombic; space group <i>Pnma</i>; <i>Z</i> = 4) and compared it with the known structure of Ag(O<sub>3</sub>SCH<sub>2</sub>Br) (Charbonnier, F.; Faure, R.; Loiseleur, H. <i>Acta Crystallogr., Sect. B </i><b>1978</b>,<i> 34</i>, 3598−3601). The halogen atom in each is apical (three-coordinate), being weakly coordinated to two silver ions. <sup>127</sup>I NQR studies on Ag(O<sub>3</sub>SCH<sub>2</sub>I) show the expected NQR consequences of three-coordination of iodine:  substantially reduced NQR frequencies ν<sub>1</sub> and ν<sub>2</sub> and a fairly small NQR asymmetry parameter η. The reduction of the halogen NQR frequency of the coordinating halogen atom in Ag(O<sub>3</sub>SCH<sub>2</sub>X) becomes more substantial in the series X = Cl < Br < I, indicating that the coordination to Ag<sup>+</sup> strengthens in this series, as expected from hard−soft acid−base principles. The numbers of electrons donated by the organic iodine atom to Ag<sup>+</sup> have been estimated; these indicate that the bonding to the cation is weak but not insignificant. We have not found any evidence for the bonding of these organohalogen atoms to another soft-acid metal ion, thallium. A scheme for recycling of thallium halide wastes is included.