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The spin crossover compound Fe(H 2 B(pyrazole)(pyridylpyrazole)) 2 was investigated in detail on Ag(111) with scanning tunneling microscopy (STM). A large fraction of the deposited molecules condenses into gridlike tetramers. Two molecules of each tetramer may be converted between two states by current injection. We attribute this effect to a spin transition. This interpretation is supported by control experiments on the analogous, magnetically passive Zn compound that forms virtually identical tetramers but exhibits no switching. The switching yields were studied for various electron energies, and the resulting values exceed those reported from other SCO systems by 2 orders of magnitude. The other two molecules of a tetramer were immutable. However, they may be used as contacts for current injection that leads to conversion of one of their neighbors. This "remote" switching is fairly efficient with yields reduced by only one to two orders of magnitude compared to direct excitation of a switchable molecule. We present a model of the tetramer structure that reproduces key observations from the experiments. In particular, sterical blocking prevents spin crossover of two molecules of a tetramer. Density functional theory calculations show that the model indeed represents a minimum energy structure. They also reproduce STM images and corroborate a remote-switching mechanism that is based on electron transfer between molecules.

Two polymorphic modifications ( 1-I and 1-II ) of the new spin crossover (SCO) complex [Fe{H 2 B(pz)(pypz)} 2 ] (pz = pyrazole, pypz = pyridylpyrazole; 1 ) were prepared and investigated by differential scanning calorimetry (DSC), magnetic measurements, Mößbauer, vibrational, and absorption spectroscopy as well as single-crystal and X-ray powder diffraction. DSC measurements reveal that upon heating the thermodynamically metastable form 1-II to ∼178 °C it transforms into 1-I in an exothermic reaction, which proves that these modifications are related by monotropism. Both forms show thermal SCO with T 1/2 values of 390 K ( 1-II ) and 270 K ( 1-I ). An analysis of the crystal structures of 1-II and the corresponding Zn(II) ( 2 ) and Co(II) ( 3 ) complexes that are isotypic with 1-I reveals that form II consists of dimers coupled by strong intramolecular π···π interactions, which is not the case for 1-I . In agreement with these findings, investigations of thin films of 1 , where significant π···π interactions should be absent, reveal SCO behavior similar to that of 1-I . These results underscore the importance of cooperativity for the spin-transition behavior of this class of complexes.