Explanation of the anomalous complexation of silver(I) with ammonia in terms of the poor affinity of the ion for water

Silver(I) complexes with ammonia show the anomalous property that in aqueous solution the second association constant is larger than the first. This effect is known not to arise from gas-phase energetic effects or from any known mechanism through which similar anomalous behavior has been observed for other systems. Using density-functional theory (DFT) calculations for the inner-shell gas-phase complexes of Cu(I), Ag(I), and Au(I) with ammonia and/or aquo ligands, we conclude that the anomalous behavior of Ag(l) arises from its poor affinity for the solvent. Addition of the first ammonia results in the exclusion of not one but two water molecules from the inner solvation shell, selectively reducing the equilibrium constant for the first association. This is a strong specific solvation effect that cannot be described using dielectric continuum models of solvation.