Silver(I) Complexes of the Weakly Coordinating Solvents SO2 and CH2Cl2: Crystal Structures, Bonding, and Energetics of [Ag(OSO)][Al{OC(CF3)(3)}(4)], [Ag(OSO)(2/2)][SbF6], and [Ag(CH2Cl2)(2)][SbF6]

The synthetically useful solvent-free silver(I) salt Ag[Al(pftb)(4)] (pftb=-OC(CF3)(3)) was prepared by metathesis reaction of Li[Al(pftb)(4)] with Ag[SbF6] in liquid SO2. The solvated complexes [Ag(OSO)][Al(pftb)(4)], [Ag(OSO)(2/2)][SbF6], and [Ag(CH2Cl2)(2)][SbF6] were prepared and isolated by special techniques at low temperatures and structurally characterized by single-crystal X-ray diffraction. The SO2 complexes provide the first examples of coordination of the very weak Lewis base SO2 to silver(I). The SO2 molecule in [Ag(OSO)][Al(pftb)(4)] is eta(1)-O coordinated to Ag+, while the SO2 ligands in [Ag(OSO)(2/2)][SbF6] bridge two Ag+ ions in an eta(2)-O,O' (trans,trans) manner. [Ag(CH2Cl2)(2)][SbF6] contains [Ag(CH2Cl2)(2)](+) ions linked through [SbF6](-) ions to give a polymeric structure. The solid-state silver(I) ion affinities (SIA) of SO2 and CH2Cl2, based on bond lengths and corresponding valence units in the corresponding complexes and tensimetric titrations of Ag[Al(pftb)(4)] and Ag[SbF6] with SO2 vapor, show that SO2 is a weaker ligand to Ag+ than the commonly used weakly coordinating solvent CH2Cl2 and indicated that binding strength of SO2 to silver(I) in the silver(I) salts increases with increasing size of the corresponding counteranion ([Al(pftb)(4)](-)>[SbF6](-)). The experimental findings are in good agreement with theoretical gas-phase ligand-binding energies of [Ag(L)(n)](+) (L = SO2, CH2Cl2; n = 1, 2) and solid-state enthalpies obtained from Born-Fajans-Haber cycles by using the volume-based thermodynamics (VBT) approach. Bonding analysis (VB, NBO, MO) of [Ag(L)(n)](+) suggests that these complexes are almost completely stabilized by electrostatic interaction, that is, monopole-dipole interaction, with almost no covalent contribution by electron donation from the ligand orbitals into the vacant 5s orbital of Ag+. All experimental findings and theoretical considerations demonstrate that SO2 is less covalently bound to Ag+ than CH2Cl2 and support the thesis that SO2 is a polar but non-coordinating solvent towards Ag+.