Transition metals spin crossover

Giitlich P, Goodwin HA (2004) Spin crossover in transition metal compounds I, II and III. Springer, Berlin Heidelberg New York... 

Real JA (1999) Bistability in iron(II) spin crossover systems a supramolecular function. In Sauvage JP (ed) Transition metals in supramolecular chemistry. Wiley, p 53... 

The branched tetradentate ligand tris(2-aminoethyl)amine (tren) forms rather stable metal complexes with most transition metal ions. It is a very hard and basic ligand and consequently its iron(II) complexes are all high-spin. Later we will discuss hexadentate derivatives of this ligand which form crossover complexes (see Sect. 3.2). 

Martin et al. have developed a unique series of capped tris(l,2-di-aminoethane) cages which can encapsulate divalent transition metal ions in a near octahedral geometry (28). The iron(II) complex with the ligand (NH2)2sar turns out to be a crossover system in solution [34], but the solid triflate salt is low-spin [43]. This is the only Fe(II) crossover system having 6 identical aliphatic nitrogen donors. 

If the coordination number of a given complex of a first row transition metal ion exceeds six there seems to be a general stabilization of the high-spin configuration. To our knowledge, there are no examples of Fe(II) crossover complexes with such high coordination numbers. 

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]. 

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