Spin crossover

    The choice between a low- and high-spin configuration for d⁴, d⁵, d⁶ and d⁷ complexes is not always unique and a spin crossover sometimes occurs; this may be initiated by a change in pressure (e.g. a low- to high spin crossover for [Fe(CN)₅(NH₃)]^3- at high pressure) or temperature (e.g. octahedral [Fe(phen)₂(NCS-N)₂], octahedral [Fe(1.1)₂] and the square-based pyramidal complex 1.2 undergo low- to high-spin crossovers at 175, 391 and 180K respectively). The change in the value of μ_eff which accompanies the spin crossover may be gradual or abrupt (Figure 1.1).

Fig. 1.1 The dependence of the observed values of μ_eff on temperature for (a) [Fe(phen)₂(NCS-N(₂] where low- to high-spin crossover occurs abruptly at 175 K, and (b) [Fe(btz)₂(NCS-N(₂] where low- to high-spin crossover occurs more gradually. Ligand abbreviations are defined in the figure [data: J.-A. Real et al. (1992) Inorg. Chem., vol. 31, p. 4972].

(1.1)                                                                (1.2)