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.