  Charge transport through single molecules can be influenced by the charge and
spin states of redox-active metal centres placed in the transport pathway.
These molecular intrinsic properties are usually addressed by varying the
molecules electrochemical and magnetic environment, a procedure that requires
complex setups with multiple terminals. Here we show that oxidation and
reduction of organometallic compounds containing either Fe, Ru or Mo centres
can solely be triggered by the electric field applied to a two-terminal
molecular junction. Whereas all compounds exhibit bias-dependent hysteresis,
the Mo-containing compound additionally shows an abrupt voltage-induced
conductance switching, yielding high to low current ratios exceeding 1000 at
voltage stimuli of less than 1.0 V. DFT calculations identify a localized,
redox active molecular orbital that is weakly coupled to the electrodes and
closely aligned with the Fermi energy of the leads because of the
spin-polarised ground state unique to the Mo centre. This situation opens an
additional slow and incoherent hopping channel for transport, triggering a
transient charging effect of the entire molecule and a strong hysteresis with
unprecedented high low-to-high current ratios.
