Weakly coordinating anions: Crystallographic and NQR studies of halogen-metal bonding in silver, thallium, sodium, and potassium halomethanesulfonates

Cl-35, Br-79.81, and I-127 NOR (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) Angstrom; b = 7,544(2) Angstrom; c = 4.925(2) Angstrom; 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. I-127 NOR studies on Ag(O3SCH2I) show the expected NOR consequences of three-coordination of iodine: substantially reduced NOR frequencies nu(1) and nu(2) and a fairly small NOR asymmetry parameter eta. The reduction of the halogen NOR 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.