Spin-crossover phenomenon transition temperatures

About twenty years ago we reported on the di-isothiocyanato iron(II) complex of the tetradentate ligand tpa (tris(2-pyridylmethyl)amine) [7] (6). It was shown that this complex exhibits the spin crossover phenomenon with a critical temperature Tm of about 170 K. Several different solvated phases of the same system have since been characterized by Chansou et al. [8]. The unsolvated phase which can be isolated from an aqueous solution has been investigated by nuclear forward scattering (NFS), nuclear inelastic scattering (NIS) [9], extended x-ray absorption fine structure (EXAFS) spectroscopy, conventional Mossbauer spectroscopy, and by measurements of the magnetic susceptibility (SQUID) [10-13]. The various measurements consistently show that the transition is complete and abrupt and it exhibits a hysteresis loop between 102 and 110 K. 

The thermally induced spin crossover phenomenon found in the solid state, as well as in solutions of the complexes of iron(II), iron(III) and cobalt(II), is a very promising, relatively new research field. In these complexes, the spin state of the central atom changes from high spin to low spin when the temperature is lowered. This phenomenon is expected to find application in the treatment of some tumors by hyperthermy. A transition metal complex is expected to be designed in such a way that its upper-limit transition temperature corresponds to the temperature desired for the treatment of some tumors by hyperthermy. It should be expected that the crossover phenomenon will attract a number of researchers. 

The spin crossover (SCO) phenomenon was studied extensively over the last 50 years (26). Compounds that exhibit a transition between the LS and HS states upon changes in temperature, pressure, or irradiation with light are attractive candidates for the development of magnetic sensors and memory devices (27). Most of the SCO compounds documented in the literature are Fe(II) complexes. An examination of the coordination environment of Fe(II) ions in many of these structures, includingthe very first Fe(II) SCO complex, [Fe (phen)2(NCS)2] (272), suggests that spin transition also can be observed in compounds where CN acts as a bridge between metal ions and is N bound to the Fe(II) center. 

Transition metal complexes of the 3d" type (n = 4, 5, 6, or 7), for which the spin state of the central metal atom changes from high-spin to low-spin when the temperature is lowered, constitute a phenomenon that has been much discussed lately.A review article of Giitlich et al. citing 192 references covers the field thoroughly until 1993. The phenomenon is accom-" panied by changes of magnetic and optical properties, which enable the so-called spin crossover (or spin transition) to be easily followed. 

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