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Date: 19-6-2019
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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 μeff which accompanies the spin crossover may be gradual or abrupt (Figure 1.1).
Fig. 1.1 The dependence of the observed values of μeff 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)
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