Nb(2)©Au(6): a molecular wheel with a short Nb[triple bond, length as m-dash]Nb triple bond coordinated by an Au(6) ring and reinforced by σ aromaticity

We report a photoelectron spectroscopy and high-resolution photoelectron imaging study of a bimetallic Nb(2)Au(6)(–) cluster. Theoretical calculations, in conjunction with the experimental data, reveal that Nb(2)Au(6)(–/0) possess high-symmetry D(6h) structures featuring a Nb–Nb axis coordinated equatorially by an Au(6) ring. Chemical bonding analyses show that there are two π bonds and one σ bond in the Nb(2) moiety in Nb(2)©Au(6), 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(2). The totally delocalized σ bonding in Nb(2)©Au(6) is reminiscent of σ aromaticity, representing a new bonding mode in metal–ligand systems. The unusually short Nb–Nb bond length in Nb(2)©Au(6) shows that the Au(6) ring can serve as a bridging ligand to facilitate multiple bonding in transition metal dimers via delocalized σ bonding.