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 den sity functional theory. It was found that they correlate linearly with expe rimental proton affinities, a contradiction to the HSAB principle as H+ is a 'hard' acid, and Ag+. is a 'soft' acid. This linear correlation between t he Ag+ and proton affinities diminishes as the number of R-CN ligands attac hed 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 expectati on that the electron donating or withdrawing properties of the R group beco me much less important than in the previous two steps, where the positive c harge on the metal ion is not well delocalized. Delocalization of the charg e of a metal ion occurs only when a sufficiently large number of ligands ar e attached to the metal ion. When this condition is satisfied, our data sug gests that the HSAB principle may be applicable. (C) 2001 Elsevier Science B.V. All rights reserved.