Synthesis and Characterization of Thermochromic Metal Complexes with a Near Room Temperature High-Spin to Low-Spin Crossover

For an undergraduate inorganic curriculum that includes transition metals chemistry, a discussion of high-spin and low-spin metal complexes is often covered. Traditional texts typically teach that the dominant spin state is the result of the metal’s oxidation state along with the nature of the ligand, implying the electronic arrangement is a static phenomenon. However, according to the Maxwell-Boltzmann theory, the valance electronic distribution will be temperature dependent; for a given metal complex the probability of pairing electrons, and therefore the spin state, will also depend on temperature. As a result, there exists a temperature at which the dominant spin state of the complex changes. Here we present a laboratory experiment that can be used in a foundational inorganic chemistry course that includes the synthesis and characterization of a set of iron(II) metal complexes that illustrates the concept of a spin switch or spin crossover process at near ambient temperature. We present an electronic absorption based approach in order to characterize this phenomenon and determine the thermodynamic parameters associated with this process. Various combinations of compounds synthesized and characterization approaches can be employed depending on what instrumentation is available at a given institution.