The digold molecule Au ₂ and its ions Au ₂ ⁺ , Au ₂ ¯ and Au ₂ ²⁺ : models and protagonists

Abstract

Abstract The electronic structures, bonding characteristics, and catalytic properties of the gold dimers Au 2 , Au 2 + , Au 2 ¯, and Au 2 2+ have been thoroughly studied using advanced gas-phase experimental techniques. Quantum chemical calculations reveal the significant influence of relativistic effects – including spin-orbit coupling – and electron correlation, emphasizing the complexity of their electronic character. Combined spectroscopic and computational studies have determined spectroscopic constants, bond lengths, dominant electronic configurations, and bond orders in both ground and excited states, although the full range of electronic transitions and relaxation mechanisms remains incompletely assigned. Photoexcitation can strengthen the metal-metal bond. In neutral Au 2 , the Au–Au bond exhibits notable charge-shift character. Surface analyses reveal “σ-holes” and “σ-humps”, suggesting potential for regium, hydrogen, and halogen bonding at specific molecular sites. Despite having a “magic number” of two valence electrons, Au 2 does not meet all criteria for classification as a superatom. The energetics of bond formation between two gold atoms – or between Au and Au + – have yet to be reconciled with different theoretical models. The singly charged digold cation and anion selectively form adducts with various neutral molecules and serve as efficient catalysts or precatalysts, for which well-defined catalytic cycles have been proposed. However, some mechanistic interpretations remain contentious due to conflicting experimental results. Theoretical evidence indicates that the experimentally observed metastable digold dication exists exclusively as the excimer ion [Au 2 2+ ]*.