Coordination and Solvation in Gas-Phase Ag⁺(C₂H₂) Complexes Studied with Selected-Ion Infrared Spectroscopy

Silver-acetylene cation complexes of the form Ag⁺(C₂H₂) (n = 1-9) were produced via laser ablation in a supersonic expansion of acetylene/argon. The ions were mass selected and studied via infrared laser photodissociation spectroscopy in the C-H stretching region (3000-3500 cm^-1). Fragmentation patterns indicate that four ligands are strongly coordinated to the metal cation. Density functional theory calculations were performed in support of the experimental data. Together, infrared spectroscopy and theory provide insight into the structure and bonding of these complexes. The Ag⁺(C₂H₂) (n = 1-4) species are shown to be η²-bonded, cation-π complexes with red-shifted C-H stretches on the acetylene ligands. Unlike Cu⁺(C₂H₂) and Au⁺(C₂H₂) complexes, which have a maximum coordination of three, silver cation is tetrahedrally coordinated to four acetylene ligands. Larger complexes (n = 5-9) are formed by solvation of the Ag⁺(C₂H₂)₄ core with acetylene. Similar to Cu⁺(C₂H₂) and Au⁺(C₂H₂) complexes, acetylene solvation leads to new and interesting infrared band patterns that are quite distinctive from those of the smaller complexes.