When does the Hard and Soft Acid Base principle apply in the gas phase?

We have studied the silver ion affinities of several RCN ligands using density functional theory. It was found that they correlate linearly with experimental proton affinities, a contradiction to the HSAB principle as H+ is a ‘hard’ acid, and Ag+ is a ‘soft’ acid. This linear correlation between the Ag+ and proton affinities diminishes as the number of RCN ligands attached to the Ag+ ion increases from one to three. In the third addition step, the silver ion affinities nearly level off in agreement with the expectation that the electron donating or withdrawing properties of the R group become much less important than in the previous two steps, where the positive charge on the metal ion is not well delocalized. Delocalization of the charge of a metal ion occurs only when a sufficiently large number of ligands are attached to the metal ion. When this condition is satisfied, our data suggests that the HSAB principle may be applicable.