Infrared spectra and structures of the stable CuH2-, AgH2-, AuH2-, and AuH4- anions and the AuH2 molecule

Gold is noble, but excited gold is reactive. Reactions of laser-ablated copper, silver, and gold with H-2 in excess argon, neon, and pure hydrogen during condensation at 3.5 K give the MH molecules and the (H-2)MH complexes as major products and the MH2- and AuH4- anions as minor products. These new molecular anions are identified from matrix infrared spectra with isotopic substitution (HD, D-2, and H-2 + D-2) and comparison to frequencies calculated by density functional theory. The stable linear MH2- anions are unique in that their corresponding neutral MH2 molecules are higher in energy than M + H-2 and thus unstable to M + H-2 decomposition. Infrared spectra are observed for the bending modes of AuH2, AuHD, and AuD2 in solid H-2, HD, and D-2, respectively. The observation of square-planar AuH4- attests the stability of the higher Au(III) oxidation state for gold. The synthesis of CuH2- in solid compounds has potential for use in hydrogen storage.