Bond-bending isomerism of Au(2)I(3)(–): competition between covalent bonding and aurophilicity

We report a joint photoelectron spectroscopy and theoretical investigation of the gaseous Au(2)I(3)(–) 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°), while aurophilic interactions pull the two Au atoms much closer, leading to an acutely bent structure (72.0°) with an Au–Au distance of 3.08 Å. 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 co-existence of a combination of isomers with both acute and obtuse bending angles at room temperature. The two bond-bending isomers of Au(2)I(3)(–) reveal a unique example of one molecule being able to oscillate between different structures as a result of two competing chemical forces.