Bond-bending isomerism of Au2I3-: competition between covalent bonding and aurophilicity

We report a joint photoelectron spectroscopy and theoretical investigation of the gaseous Au2I3- cluster, which is found to exhibit two types of isomers due to competition between Au-I covalent bonding and Au-Au aurophilic interactions. The covalent bonding favors a bent IAuIAuI- structure with an obtuse Au-I-Au angle (100.7 degrees), while aurophilic interactions pull the two Au atoms much closer, leading to an acutely bent structure (72.0 degrees) with an Au-Au distance of 3.08 angstrom. The two isomers are separated by a small barrier and are nearly degenerate with the obtuse isomer being slightly more stable. At low temperature, only the obtuse isomer is observed; distinct experimental evidence is observed for the coexistence of a combination of isomers with both acute and obtuse bending angles at room temperature. The two bond-bending isomers of Au2I3- reveal a unique example of one molecule being able to oscillate between different structures as a result of two competing chemical forces.