Binding energies of silver ion ligand, L, complexes AgL2+ determined from ligand-exchange equilibria in the gas phase

The free energy Delta G degrees, entropy Delta S degrees, and enthalpy Delta H degrees for the reaction: AgL2+ = Ag+ + 2L in the gas phase were determined for 16 different ligands L which included oxygen bases such as H2O, MeOH, EtOH, Me2CO, and Me2SO, nitrogen bases NH3 and MeCN, and sulfur bases Me2S. The determinations were based on measurements of the exchange equilibria AgA(2)(+) + B = AgAB(+) + A and AgAB(+) + B = AgB2 + A where A and B are different ligands L. The exchange equilibria were determined in a "high"-pressure ion source at 10 Torr bath gas containing A and B in the 10-100 mTorr range and using Ag(MeOH)(2)(+) ions produced by electrospray. A scale of Delta G degrees values for the exchange reaction AgA(2)(+) + 2B = AgB2+ + 2A was established and calibrated to the Delta G degrees for Ag(H2O)(2)(+) = Ag+ + 2H(2)O, due to Holland and Castleman, obtaining thus absolute Delta G degrees values for all ligands involved. Theoretically calculated Delta S degrees values led to Delta H degrees. Comparison of the Delta G degrees and Delta H degrees results with binding energies for the same ligands in complexes with the alkali ions Li+ and K+ showed that while a good correlation is observed for the alkali ions as a group, only a very poor correlation is observed between the alkali ions and Ag+. In particular, the "soft" base Me2S showed, relative to the "hard" oxygen bases, a much stronger bonding to Ag+. The comparison was extended to CuL2+ on the basis of additional exchange equilibria determinations. The CuL2+ and AgL2+ results combined with earlier determinations of MnL2+, CoL2+ and CuL2+ by Jones and Staley provide a partial confirmation of the hard and soft acid-base (HSAB) concept for these systems. However, a more detailed comparison on the basis of the absolute hardness, eta, scale developed by Pearson and co-workers indicates only a very limited agreement when a larger variety of bases is included.