Spin transition curves

Fig. la-d Representation of the principal types of spin transition curves (high spin fraction (7hs) (y axis) vs temperature (T) (x axis) a gradual b abrupt c with hysteresis d two-step e incomplete... 

Despite the relative lack of predictability, the number of systems now known to display a spin transition curve of type (c) is remarkably high, and highest for iron(II) where, significantly, the change in intramolecular dimensions is the greatest for the ions for which SCO is relatively common (Fe(II), Fe(III), Co(II)). 

DSC measurements with a microcalorimeter played a key role in tracing the origin of the step observed in the spin transition curve of [Fe(2-pic)3]-Cl2-EtOH [24]. The mixing entropy derived from the measured heat capacity data showed a significant reduction in the region of the step. This has been... 

EXAFS (Extended X-ray Absorption Fine Structure) measurements using synchrotron radiation have been successfully applied to the determination of structural details of SCO systems and have been particularly useful when it has not been possible to obtain suitable crystals for X-ray diffraction studies. Perhaps the most significant application has been in elucidating important aspects of the structure of the iron(II) SCO linear polymers derived from 1,2,4-triazoles [56]. EXAFS has also been applied to probe the dimensions of LIESST-generated metastable high spin states [57]. It has even been used to generate a spin transition curve from multi-temperature measurements [58]. 

Somewhat unusual pressure dependence of the nature of the spin transition curve has been found for chain-like SCO systems containing substituted bridging triazole ligands [163, 164]. Although the transition is displaced to higher temperatures with increase in pressure, the shape of the transition curve, unusually, is effectively constant, i.e. there is no significant change in the hysteresis width and the transition remains virtually complete. This has been taken to indicate that the cooperativity associated with the transitions in these and related systems is confined within the iron(II) triazole chains. 

The spin transition curve (Fig. 1) for the triflate monohydrate displays some remarkable features [91]. The HS—>LS transition is particularly abrupt... 

Abstract This review reports on the study of the interplay between magnetic coupling and spin transition in 2,2 -bipyrimidine (bpym)-bridged iron(II) dinuclear compounds. The coexistence of both phenomena has been observed in [Fe(bpym)(NCS)2]2(bpym), [Fe(bpym)(NCSe)2]2(bpym) and [Fe(bt)(NCS)2]2(bpym) (bpym = 2,2 -bipyrimidine, bt = 2,2 -bithiazoline) by the action of external physical perturbations such as heat, pressure or electromagnetic radiation. The competition between magnetic exchange and spin crossover has been studied in [Fe(bpym)(NCS)2]2(bpym) at 0.63 GPa. LIESST experiments carried out on [Fe(bpym)(NCSe)2]2(bpym) and [Fe(bt)(NCS)2]2(bpym) at 4.2 K have shown that it is possible to generate dinuclear molecules with different spin states in this class of compounds. A special feature of the spin crossover process in the dinuclear compounds studied so far is the plateau in the spin transition curve. Up to now, it has not been possible to explore with a microscopic physical method the nature of the species... 

As mentioned above, the particular characteristics of the spin crossover process in dinuclear compounds is the appearance of a plateau in the spin transition curve. From the analysis of the results of the pressure experiments, it is inferred that the plateau results from successive ST in the two metal centres, leading first to the formation of relatively stable [HS-LS] pairs and then, above a critical pressure, to the formation of [LS-LS] pairs on further lowering of the temperature. The intermolecular interactions between [HS-LS] pairs leads to domains that contribute to the stability of the... 

The special feature of the spin crossover process in all bpym-bridged dinuclear compounds studied so far is the occurrence of a plateau in the spin transition curve. A reasonable assumption to account for this observation is that a thermal spin transition takes place successively in the two metal centres. However, it cannot be excluded that spin transition takes place simultaneously in the dinuclear units leading directly from [HS—HS] pairs to [LS-LS] pairs with decreasing temperature. Therefore, two possible conversion pathways for [HS—HS] pairs with decreasing temperature may be proposed [HS—HS]<->[HS—LS]<->[LS—LS] or [HS-HS] [LS-LS]. The differentiation of the existence of the [LS—LS], [HS—LS], and [HS—HS] spin pairs is not trivial and has recently been solved experimentally by utilisation of magnetisation versus magnetic field measurements as a macroscopic tool [9], and by Mossbauer spectroscopy in an applied magnetic field as a microscopic tool [11]. 

The inter-conversion of the spin states in many instances is so rapid that the separate contributions to the 57Fe Mossbauer spectra are not resolved. Thus this technique, which has proved so diagnostic in iron(II) systems, is frequently less suited to the derivation of spin transition curves for iron(III). A further corollary of the faster spin state inter-conversion is the rarity of the LIESST effect among iron(III) systems, in contrast to its ubiquitous occurrence in iron(II). 

Figure 17-E-4 Classification of spin transition curves (a) gradual, (b) abrupt, (c) discontinuous with hysteresis, (d) two-step, (e) incomplete spin transition.

Incorporation of hydrogen bonding centers within the Kgand leading to a network of interactions with the associated anions and/or solvent molecules has been achieved in a number of systems. The classic example is the 2-picolylamine (pic) system, [Fe(pic)3]X2.(solvent) , in which the nature of the spin transition curve depends on both X and the solvent e.g., the chloride... 

---