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

The free energy Δ G °, entropy Δ S °, and enthalpy Δ H ° for the reaction: AgL 2 + = Ag + + 2L in the gas phase were determined for 16 different ligands L which included oxygen bases such as H 2 O, MeOH, EtOH, Me 2 CO, and Me 2 SO, nitrogen bases NH 3 and MeCN, and sulfur bases Me 2 S. The determinations were based on measurements of the exchange equilibria AgA 2 + + B = AgAB + + A and AgAB + + B = AgB 2 + 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 Δ G ° values for the exchange reaction AgA 2 + + 2B = AgB 2 + + 2A was established and calibrated to the Δ G ° for Ag(H 2 O) 2 + = Ag + + 2H 2 O, due to Holland and Castleman, obtaining thus absolute Δ G ° values for all ligands involved. Theoretically calculated Δ S ° values led to Δ H °. Comparison of the Δ G ° and Δ H ° 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 Me 2 S showed, relative to the “hard” oxygen bases, a much stronger bonding to Ag + . The comparison was extended to CuL 2 + on the basis of additional exchange equilibria determinations. The CuL 2 + and AgL 2 + results combined with earlier determinations of MnL 2 +, CoL 2 + and CuL 2 + 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.