Three-State Switching of an Fe Spin Crossover Complex

Isolated tetramers of the spin crossover compound Fe(H 2 B(pyrazole)(pyridylpyrazole)) 2 were investigated on Ag(111) at cryogenic temperature (5 K) with a scanning tunneling microscope (STM) in ultrahigh vacuum. In addition to spin states obtained by electron-induced spin crossover, a third state of the complexes is discovered with different current–voltage characteristics and intramolecular image contrast. It is stable over a limited range of voltages beyond which it decays into the low- and high-spin states of the Fe complex. The state may be prepared in a deterministic manner that enables measurements of its decay dynamics at various currents and voltages. We interpret this state as being due to a negatively charged Fe complex in its high-spin state, which is isoelectronic to the analogous Co compound.