Thiol-tethered Ru-II terpyridine complexes were synthesized for a voltage-driven molecular switch and used to understand the switch-on mechanism of the molecular switches of single metal complexes in the solid-state molecular junction in a vacuum. Molecularly resolved scanning tunneling microscopy (STM) images revealed well-defined single Ru-II complexes isolated in the highly ordered dielectric monolayer. When a negative sample-bias was applied, the threshold voltage to the high conductance state in the molecular junctions of the Ru-II complex was consistent with the electronic energy gap between the Fermi level of the gold substrate and the lowest ligand-centered redox state of the metal complex molecule. As an active redox center leading to conductance switching in the molecule, the lowest ligand-centered redox state of Ru-II complexes was suggested to trap an electron injected from the gold substrate. Our suggestions for a single-molecule switch-on mechanism in the solid state can provide guidance in a design that improves the charge-trapping efficiency of the ligands with different metal substrates.