Formation of Metallocene Single-Molecule Junctions Via Metal-Metal Bonds

We study single-molecule junction formation of group VIII metallocenes─ferrocene, ruthenocene, and osmocene─with gold (Au) electrodes using the scanning tunneling microscope-based break junction technique. Unlike ferrocene, both ruthenocene and osmocene can form molecular junctions under ambient conditions without chemical linkers. We propose that Au electrodes bind to the metal center and one of the cyclopentadienyl (Cp) rings via a ring-slippage process, forming a molecular junction. Control measurements demonstrate that the metal centers bind to uncoordinated Au exclusively in the +3 oxidation state. Ab initio quantum transport calculations corroborate this mechanism for metallocene junction formation. This work highlights the formation of metal-metal (Ru-Au and Os-Au) bonds in metallocene-based single-molecule devices, challenging the assumption that metallocenes bind exclusively through van der Waals interactions between the Cp ring and the Au electrode. Our findings introduce a method for creating organometallic single-molecule devices with metal-metal bonds, enabling more stable and versatile molecular electronics.