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2. Fe(phen)2(SCN)2 has an effective magnetic moment of ~5.5 B.M. at 300 k and ~0.90 B.M. at 150 k. Explain this difference in magnetic moments and comment on any differences in the electronic spectra at the two temperatures
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The observed change in magnetic moment of Fe(phen)2(SCN)2 with temperature suggests a spin-crossover phenomenon. At higher temperatures (300 K), the iron(II) ion is likely in a high-spin state (S=2) with four unpaired electrons, leading to a magnetic moment of ~5.5 B.M. As the temperature decreases to 150 K, the iron(II) ion transitions to a low-spin state (S=0) with no unpaired electrons, resulting in a much lower magnetic moment of ~0.90 B.M. This transition is driven by the energy difference between the high-spin and low-spin states, which is influenced by the ligand field strength and temperature.
In terms of the electronic spectra, at 300 K, the high-spin state would exhibit a weaker ligand field, leading to lower energy d-d transitions and potentially a broader, less intense absorption band. At 150 K, the low-spin state would exhibit a stronger ligand field, resulting in higher energy d-d transitions and potentially a narrower, more intense absorption band. However, specific spectral changes would depend on the individual electronic transitions involved.
This behavior is typical of certain transition metal complexes with labile coordination environments and a delicate balance between ligand field strength and temperature.
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