Bonding and Acidity of the Formal Hydride in the Prototypical Au₉ (PPh₃ )₈ H²⁺ Nanocluster

The role of hydrogen atoms as surface ligands on metal nanoclusters is of profound importance but remains difficult to directly study. While hydrogen atoms often appear to be incorporated formally as hydrides, evidence suggests that they donate electrons to the cluster's delocalized superatomic orbitals and may consequently behave as acidic protons that play key roles in synthetic or catalytic mechanisms. Here we directly test this assertion for the prototypical Au₉ (PPh₃ )₈ H²⁺ nanocluster, formed by addition of a hydride to the well-characterized Au₉ (PPh₃ )₈ ³⁺ . Using gas-phase infrared spectroscopy, we were able to unambiguously isolate Au₉ (PPh₃ )₈ H²⁺ and Au₉ (PPh₃ )₈ D²⁺ , revealing an Au-H stretching mode at 1528 cm^-1 that shifts to 1038 cm^-1 upon deuteration. This shift is greater than the maximum expected for a typical harmonic potential, suggesting a potential governing cluster-H bonding that has some square-well character consistent with the hydrogen nucleus behaving as a metal atom in the cluster core. Complexing this cluster with very weak bases reveals a redshift of 37 cm^-1 in the Au-H vibration, consistent with those typically seen for moderately acidic groups in gas phase molecules and providing an estimate of the acidity of Au₉ (PPh₃ )₈ H²⁺ , at least with regard to its surface reactivity.