Electrically tunable molecules are highly attractive for the construction of molecular devices, such as switches, transistors, or machines. Here, we present a novel nanomechanical element triggered by an electrical bias as external stimulus. We demonstrate that a transitory chemical bond between a copper atom and coordinating organic molecules adsorbed on a metal surface acts as variable frequency switch, which can be actuated and probed by means of low-temperature scanning tunneling microscopy. Whereas below a threshold bias voltage the bond is permanently either formed or broken the bonding state continuously oscillates at higher voltages. The switching rate of the bistable molecular system can be widely tuned from below 1 Hz up to the kilohertz regime. The quantum yield per tunneling electron to trigger a transition between the two states varies spatially and is related to the local density of states of the bonded and nonbonded configuration.