Nb2©Au6: a molecular wheel with a short NbNb triple bond coordinated by an Au6 ring and reinforced by σ aromaticity†
We report a photoelectron spectroscopy and high-resolution photoelectron imaging study of a bimetallic Nb2Au6− cluster. Theoretical calculations, in conjunction with the experimental data, reveal that Nb2Au6−/0 possess high-symmetry D6h structures featuring a Nb–Nb axis coordinated equatorially by an Au6 ring. Chemical bonding analyses show that there are two π bonds and one σ bond in the Nb2 moiety in Nb2©Au6, as well as five totally delocalized σ bonds. The NbNb triple bond is strengthened significantly by the delocalized σ bonds, resulting in an extremely short Nb–Nb bond length comparable to the quintuple bond in gaseous Nb2. The totally delocalized σ bonding in Nb2©Au6 is reminiscent of σ aromaticity, representing a new bonding mode in metal–ligand systems. The unusually short Nb–Nb bond length in Nb2©Au6 shows that the Au6 ring can serve as a bridging ligand to facilitate multiple bonding in transition metal dimers via delocalized σ bonding.