Binding Energies of Silver Ion−Ligand, L, Complexes AgL2+ Determined from Ligand-Exchange Equilibria in the Gas Phase

Abstract

The free energy ΔG°, entropy ΔS°, and enthalpy ΔH° 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 AgA2+ + 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 ΔG° values for the exchange reaction AgA2+ + 2B = AgB2+ + 2A was established and calibrated to the ΔG° for Ag(H2O)2+ = Ag+ + 2H2O, due to Holland and Castleman, obtaining thus absolute ΔG° values for all ligands involved. Theoretically calculated ΔS° values led to ΔH°. Comparison of the ΔG° and ΔH° 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, η, scale developed by Pearson and co-workers indicates only a very limited agreement when a larger variety of bases is included.