Thermal spin transition

Substitution of one ligand by another can generate, or alter, spin crossover characteristics. The systems studied early provide the classic illustration of this effect. Thus [Fe(py)4(NCS)2] is high spin at room temperature and does not undergo a thermal spin transition. Substitution of two of the pyridine molecules by a phenanthroline molecule gives [Fe (phen)(py)2 (NCS)2] which does undergo a thermal transition [99, 141], as does the species in which the remaining two pyridines are substituted [Fe(phen)2 (NCS)2]. As would be expected, T1/2 for the former complex (106 K) is lower... 

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 results from Mossbauer spectroscopy in applied magnetic fields clearly prove that the spin transition in the dinuclear compounds under study proceeds via [HS-HS][HS-LS][LS-LS]. Simultaneous spin transition in both metal centres of the [HS-HS] pairs converting the dinuclear pairs directly to [LS-LS] pairs can apparently be excluded, at least in the present systems. This is quite surprising in view of the fact that the present dinuclear complexes are centrosymmetric (in other words the two metal centres have identical surroundings, and should therefore experience the same ligand field strength and, consequently, thermal spin transition should occur simultaneously in both centres). 

Some complexes show a strong interdependence between crystal structure and spin-transition features. In the series of compounds [Fe(Rtz)6](BF4)2 (Rtz = 1-alkyltetrazole) the spin crossover behavior varies with the substituent R and is strongly influenced by cooperative effects. For example, the propyl derivative shows a quantitative spin transition, which is accompanied by a first-order crystallographic phase transition in the methyl and ethyl derivatives the Fe11 complexes occupy two nonequivalent lattice sites, only one of which shows a thermal spin transition.29... 

Y. Galyametdinov, V. Ksenofontov, A. Prosvirin, 1. Ovchimiikov, G. Ivanova, P. Giitlich, W. Haase, First example of coexistence of thermal spin transition and liquid-crystal properties. Angew. Chem. Int. Ed. 40, 4269 271 (2001)... 

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