Charge and size effects in π-ligand activation: an IR spectroscopic study of gold-acetylene complexes

The electronic charge and size of metal clusters play a critical role in determining ligand activation, which is a key step in many catalytic processes. Here, the charge- and size-dependent interaction of gold clusters (Au _n ^+/-, n ≤ 4) with up to four acetylene (C₂H₂) molecules is investigated using infrared photodissociation spectroscopy of He-tagged species, probing the C-H stretching region (2850-3390 cm^-1). The IR spectra, supported by density functional theory calculations, reveal distinct trends in vibrational shifts, coordination geometries, and binding motifs that reflect the clusters' charge state and number of gold atoms. Cationic clusters activate acetylene via coordination bonds and π-backdonation. Gold cations up to n = 2 bind two acetylene ligands, while larger clusters coordinate only one. Additional molecules solvate the core cation, forming a second solvation shell. As the cluster grows, charge becomes increasingly delocalized across the ion-molecule complexes, which leads to a decrease in coordination number, weaker binding energy, and reduced acetylene activation. In contrast, anionic clusters interact only through polarization forces and quadrupole interactions, which do not lead to activation. These findings provide molecular-level insight into charge-controlled π-ligand activation and offer design principles for tailoring reactivity in charged metal complexes.