Nature of the spin crossover

The total molecular energy of a system as a function of the nuclear coordinates (the adiabatic potential) exhibits different characteristics for the LS and HS states (Fig. 9.6). 

The LS state is characterised by the adiabatic potential surface i LS(r) having a minimum ifo.Ls at a certain geometry (metal-ligand distances rLS) with some definite curvature (the second derivative representing a force constant /LS). Within this nearly parabolic function a set of vibration levels e, LS occurs, the lowest one corresponding to the zero-point vibration, e0LS. The HS state differs in these characteristics in such a way that the relationships 

Just at the transition temperature the following equation holds true 

The driving force for the LS to HS conversion is the drop of the Gibbs energy to a negative value above Tc. Whether the spin transition is observable or not depends upon the actual values of the enthalpy and entropy. Restricting ourselves to the spin-only value of the state degeneracy, we arrive at the expression 

The available experimental data for AS H/k range from 4.2 to 9.6 so that there are definitely some more contributions than pure electronic (spin and orbital) degeneracies. These may arise from the different vibration spectra of the respective LS and HS states. The adiabatic potential for the HS state is 

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Probably the most important experimental work which confirms the cooperative nature of the spin crossover transition is the heat capacity measurements on Fe(phenanthroline)2(NCS)2 and Fe(phenanthroline)2-(NCSe)2 by Sorai Seki (1974). The variation in the molar heat capacity of the latter compound with temperature is shown in Figure 3.27. Not only does the heat capacity show a sharp peak at the transition temperature (see Table 3.9), but there is also a change in the heat capacity, for the two different spin states (see Figure 3.27) at the transition. These authors propose that the total heat capacity of each spin state is made up of contributions from lattice vibrations, intramolecular vibrations, and electron thermal excitation. From this they have determined the changes in enthalpy and entropy associated with the change in spin state at the crossover. The results of these calculations are given in Table 3.9 and they... 

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... 

Cooperativity is one of the most appealing and elusive facets of the spin-crossover phenomenon. It is a main aspect because discontinuity in the magnetic and optical properties along with thermal hysteresis confer to these systems potential memory effect. Nevertheless, because most of the spin-crossover systems are discrete in nature, cooperativity stems from assemblies of molecules held together by nonco-valent interactions and, consequently, difficult to control. 

Multi-temperature X-ray diffraction data for a series of spin-crossover complexes differing in cooperativity indicates that the molecule and crystal volume variations upon spin conversion are similar in all the cases irrespective of the cooperative nature of the spin conversion [47]. So, a systematic structural analysis of specifically designed spin-crossover compounds should be of utmost importance to establish correlations between intermolecular interactions and cooperativity. The comparative structural study of [Fe(phen)2(NCS)2] and [Fe(btz)2(NCS)2] where btz = 2,2 -bi-4,5-dihydrothiazine (Figure 10) represents the sole example so far reported oriented in this direction [48,49]. It illustrates the dependence of the nature of the phenomenon on the efficiency of the intermolecular contacts in transmiting the intramolecular reorganization upon spin conversion. 

A more subtle chemical influence is the variation of the anion associated with a cationic spin crossover system, or of the nature and degree of solvation of salts or neutral species. These variations can result in the displacement of the transition temperature, even to the extent that SCO is no longer observed, or may also cause a fundamental change in the nature of the transition, for example from abrupt to gradual. The influence of the anion was first noted for salts of [Co(trpy)2]2+ [142] and later for iron(II) in salts of [Fe(paptH)2]2+ [143] and of [Fe(pic)3]2+ [127]. For the [Fe(pic)3]2+ salts the degree of completion and steepness of the ST curve increases in the order io-dide[Pg.41]

The same effect is observed for the substituted pyridyl-pyrazole and -imidazole systems. While 2-(pyrazol-l-yl)pyridine 24 gives a low spin iron(II) complex a continuous spin transition is observed centred just above room temperature in solid salts of [Fe (31)3]2+ and just below in solution [39]. Spin crossover occurs in the [Fe N6]2+ derivative of 2-(pyridin-2-yl)benzimidazole 32 (Dq(Ni2+)=1050 cm"1) but not in that of the 6-methyl-pyridyl system 33 (Dq(Ni2+)=1000 cm"1). Although the transition in salts of [Fe 323]2+ is strongly influenced by the nature of the anion and the extent of hydration, suggesting an influence of hydrogen-bonding, in all instances it is continuous [40]. 

The long-range cooperative nature of the electronic spin-state crossover in [Fe(HB(pz)3)2] and the accompanying crystallographic phase transition is... 

The N-2, N-4 bridging coordination mode has not (yet) been observed in Fe(II) spin crossover compounds, whereas the N-l, N-2 bridging mode has been confirmed by X-ray structure determinations of oligomeric and polymeric Fe(II) spin crossover materials. Depending on the nature of the substituted 1,2,4-triazole ligand and the presence of potentially coordinating an-... 

Structure determinations of several Fe(II) compounds of 2-triazolyl-l,10-phenanthroline and its methyl-substituted derivatives proved that in addition to the two nitrogen donor atoms of the 1,10-phenanthroline entity, the N-4 of the 1,2,4-triazole ring participates in coordination, even when a methyl substituent occupies the position adjacent to this donor atom [15, 16]. All compounds obtained exhibit Fe(II) spin crossover behaviour, its extent depending on the nature of the anionic groups and the solvent content. 

These complexes are either high-spin or low-spin depending on the nature of the ligand X. It has been possible to create spin-crossover complexes [20] in a few cases, such as X=NCS. ... 

Fe(III) compounds exhibiting spin crossover behaviour, its extent depending on the nature of the substituents R1 and R2 indicated in Fig. 4. 

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