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

³⁵Cl, ^79,81Br, and ¹²⁷I 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⁺ to halogen donor atoms on the surfaces of very weakly coordinating anions. We have determined the X-ray crystal structure of Ag(O₃SCH₂Cl) (a = 13.241(3) Å; b = 7.544(2) Å; c = 4.925(2) Å; orthorhombic; space group Pnma; Z = 4) and compared it with the known structure of Ag(O₃SCH₂Br) (Charbonnier, F.; Faure, R.; Loiseleur, H. Acta Crystallogr., Sect. B 1978, 34, 3598−3601). The halogen atom in each is apical (three-coordinate), being weakly coordinated to two silver ions. ¹²⁷I NQR studies on Ag(O₃SCH₂I) show the expected NQR consequences of three-coordination of iodine:  substantially reduced NQR frequencies ν₁ and ν₂ and a fairly small NQR asymmetry parameter η. The reduction of the halogen NQR frequency of the coordinating halogen atom in Ag(O₃SCH₂X) becomes more substantial in the series X = Cl < Br < I, indicating that the coordination to Ag⁺ strengthens in this series, as expected from hard−soft acid−base principles. The numbers of electrons donated by the organic iodine atom to Ag⁺ 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.