Nb©Au: a molecular wheel with a short Nb[triple bond, length as m-dash]Nb triple bond coordinated by an Au ring and reinforced by σ aromaticity.

We report a photoelectron spectroscopy and high-resolution photoelectron imaging study of a bimetallic NbAu cluster. Theoretical calculations, in conjunction with the experimental data, reveal that NbAu possess high-symmetry structures featuring a Nb-Nb axis coordinated equatorially by an Au ring. Chemical bonding analyses show that there are two π bonds and one σ bond in the Nb moiety in Nb©Au, as well as five totally delocalized σ bonds. The Nb[triple bond, length as m-dash]Nb 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 Nb. The totally delocalized σ bonding in Nb©Au is reminiscent of σ aromaticity, representing a new bonding mode in metal-ligand systems. The unusually short Nb-Nb bond length in Nb©Au shows that the Au ring can serve as a bridging ligand to facilitate multiple bonding in transition metal dimers delocalized σ bonding.