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Of the several inorganic systems that exhibit spin-state transitions, LaCoO3 and related cobaltates represent an important category of oxides exhibiting a transition from the low-spin (LS) state to a state of higher spin with increasing temperature. It was first considered that the transition was from the LS (1A1) to the high-spin (HS, 5T2) state and a variety of investigations were performed on this...
Cobalt(II), a d7 ion, can exist in a low spin doublet state or a high spin quartet state. Both spin-state configurations are known in four-, five- and six-coordinate complexes. For each of these coordination numbers, systems are known where the two spin states may be thermally interconverted. For four-coordinate and some five-coordinate systems these interconversions are best regarded as configurational...
This chapter considers thermal spin crossover occurring in transition metal compounds other than those of Fe(II), Fe(III) and Co(II). Unusual magnetic properties of several coordination and organometallic complexes of Mn(II), Mn(III), Cr(II) and Co(III) are discussed in the light of their structural features.
Valence d-orbital energies of the first row transition metals are close to the frontier π-orbital energies of o-benzoquinones. Complexes prepared with quinone ligands most commonly have the quinone coordinated with the metal in the form of a semiquinonate (SQ) radical-anion or as a catecholate (Cat) dianion. In a few unique complexes it has been possible to observe intramolecular electron transfer...
The interplay between the spin crossover and the structural properties of the complexes in the solid state is still under investigation. In particular the following questions may be asked. What are the structural modifications of the metal coordination sphere at the spin crossover? How are the dimensions and the symmetry of the crystallographic unit cell affected by the spin crossover? Conversely,...
The results of X-ray diffraction studies of spin crossover complexes of iron(II) with R=1-propyl- (ptz), 1- methyl- (mtz) and 1-ethyltetrazole (etz) ligands, [Fe(Rtz)6](BF4)2, as a function of temperature (down to 10 K) and after light-induced conversion to long-lived metastable spin states (LIESST) are reviewed. Not only has the most prominent member of this class of spin crossover compounds, viz...
The discovery of a light-induced spin transition at cryogenic temperatures in a series of iron(II) spin-crossover compounds in 1984 has had an enormous impact on spin-crossover research. Apart from being an interesting photophysical phenomenon in its own right, it provided the means of studying the dynamics of the intersystem crossing process between the high-spin and the low-spin state in a series...
We report recent work on the competition between opposite kinetic processes: (i) photo-excitation (LIESST) and relaxation processes, and (ii) direct and reverse LIESST. The light-induced instability eventually occurring at the light-induced equilibrium temperature is attributed to a cooperative origin. The subsequent light-induced thermal hysteresis (LITH) and intensity threshold effect (LIOH) are...
Mössbauer Emission Spectroscopy (MES) has been employed in studies of chemical and physical after-effects of the electron capture process 57Co(EC)57Fe in inorganic compounds. The 57Co labelled compounds are used as the Mössbauer source at variable temperatures vs a single-line absorber such as K4[Fe(CN)6]·3H2O kept at room temperature. The recorded ME spectrum yields information on the electronic...
“Ligand-driven light-induced spin change” (LD-LISC) is a photomagnetic effect based on the modulation of the ligand-field strength of a suitable spin-crossover complex through a photochemical reaction on the ligand. It allows one to switch the electronic spin state of the metal ion by means of light over a broad range of temperatures possibly including room temperature. Among the photochemical reactions...
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