SummaryThe article deals with coordination compounds of iron(II) that may exhibit thermally induced spin transition, known as spin crossover, depending on the nature of the coordinating ligand sphere. Spin transition in such compounds also occurs under pressure and irradiation with light. The spin states involved have different magnetic and optical properties suitable for their detection and characterization. Spin crossover compounds, though known for more than eight decades, have become most attractive in recent years and are extensively studied by chemists and physicists. The switching properties make such materials potential candidates for practical applications in thermal and pressure sensors as well as optical devices.The article begins with a brief description of the principle of molecular spin state switching using simple concepts of ligand field theory. Conditions to be fulfilled in order to observe spin crossover will be explained and general remarks regarding the chemical nature that is important for the occurrence of spin crossover will be made. A subsequent section describes the molecular consequences of spin crossover and the variety of physical techniques usually applied for their characterization. The effects of light irradiation (LIESST) and application of pressure are subjects of two separate sections. The major part of this account concentrates on selected spin crossover compounds of iron(II), with particular emphasis on the chemical and physical influences on the spin crossover behavior. The vast variety of compounds exhibiting this fascinating switching phenomenon encompasses mono-, oligo- and polynuclear iron(II) complexes and cages, polymeric 1D, 2D and 3D systems, nanomaterials, and polyfunctional materials that combine spin crossover with another physical or chemical property.

This article reviews the most relevant chemical and structural aspects that influence the spin-crossover phenomenon (SCO). Special attention is focussed on the recent development of SCO coordination polymers. The different approaches currently being explored in order to achieve multifunctionality in SCO materials are discussed.

The three-dimensional polymeric compounds [Fe(pz)M(CN)4]·nH2O (pz = pyrazine; M = Ni(II), Pd(II), and Pt(II)) have been synthesized and characterized. They undergo strong cooperative spin transitions, large hysteresis loops, and dramatic color changes upon spin conversion. The two-dimensional homologues [Fe(py)2M(CN)4] (py = pyridine; M = Ni(II), Pd(II), and Pt(II)) also have been synthesized and characterized. In the latter case cooperativity is smaller than in the tri-dimensional derivatives, and consequently narrower hysteresis loops were observed.

The ins and outs of spin: Using the microporous coordination polymer {Fe(pz)[Pt(CN)(4)]} (1, pz=pyrazine), incorporating spin-crossover subunits, two-directional magnetic chemo-switching is achieved at room temperature. In situ magnetic measurements following guest vapor injection show that most guest molecules transform 1 from the low-spin (LS) state to the high-spin (HS) state, whereas CS(2) uniquely causes the reverse HS-to-LS transition.

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Nanocrystals of the three‐dimensional, spin‐crossover, porous coordination framework [Fe(pz)Pt(CN)4]⋅n H2O (pz=pyrazine; n≤2.5) have been synthesized from water‐in‐oil microemulsions. The surfactant‐free nanocrystals readily desorb water and the resulting anhydrous compounds exhibit thermally induced electronic bistability accompanied by a pronounced color change (see picture; HS=high spin, LS=low spin) close to room temperature.

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