Infrared Spectroscopy of Gold Carbene Cation (AuCH₂⁺): Covalent or Dative Bonding?

The present work explores the structure of the gold carbene cation, AuCH₂⁺, using infrared multiple photon dissociation action spectroscopy and density functional theory (DFT). Unlike several other 5d transition-metal cations (M⁺ = Ta⁺, W⁺, Os⁺, Ir⁺, and Pt⁺) that react with methane by dehydrogenation to form MCH₂⁺ species, gold cations are unreactive with methane at thermal energies. Instead, the metal carbene is formed by reacting atomic gold cations formed in a laser ablation source with ethylene oxide (cC₂H₄O) pulsed into a reaction channel downstream. The resulting [Au,C,2H]⁺ product photofragmented by loss of H₂ as induced by radiation provided by the free-electron laser for intracavity experiments in the 300-1800 cm^-1 range. Comparison of the experimental spectrum, obtained by monitoring the appearance of AuC⁺, and DFT calculated spectra leads to the identification of the ground-state carbene, AuCH₂⁺ (¹A₁), as the species formed, as previously postulated theoretically. Unlike the covalent double bonds formed by the lighter, open-shell 5d transition metals, the closed-shell Au⁺ (¹S, 5d¹⁰) atom binds to methylene by donation of a pair of electrons from CH₂(¹A₁) into the empty 6s orbital of gold coupled with π back-bonding, i.e., dative bonding, as explored computationally. Contributions to the AuC⁺ appearance spectrum from larger complexes are also considered, and H₂CAu⁺(c-C₂H₄O) seems likely to contribute one band observed.