Properties of spin crossover systems

PROPERTIES OF SPIN CROSSOVER SYSTEMS 9.3.1 Factors influencing the spin crossover... 

A recent Mossbauer and magnetic susceptibility study on [Fe(phen)2ox] 5 H20 186) has identified this complex as a typical 5T2(0, ) 1Aj(Oh) spin crossover system rather than a complex with a spin triplet ground state. Moreover, it has been shown 186> that the magnetic properties are very susceptible to the amount of lattice water present Pumping off the lattice water gradually favours more and more the HS state the stable monohydrate, finally, turned out to be a paramagnet. 

Some of the polycrystalline spin crossover systems of iron(II) described above retain their spin equilibrium property upon dissolution in appropriate solvents. The Evans NMR method of measuring the change of the paramagnetic shift with temperature is the most common technique to study the magnetic behaviour of such systems. The spin transition characteristics has been observed to depend on various chemical modi-... 

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. 

The electronic effect of substituents (functional groups) in a certain stereospecific position at the ligand skeleton is well documented. For example, in [Fe(2-Y-phen)3]X2-type complexes the spin properties alter within the series of substituents as follows Y = H—low-spin complex CH30 and CH3—spin crossover systems Cl—high-spin complex. 

The formation of diimine systems by Schiff -base-type condensation of suitable aldehydes and primary amines has been widely applied. Those reported are mostly strong field systems and their relevance to the spin crossover field is generally in systems of the kind [Fe(diimine)2(NCS)2]. The effect of the incorporation of substituents likely to hinder coordination has been studied. Robinson and Busch noted a fundamental difference at room temperature in the electronic properties of the [Fe N6]2+ derivatives of 2-pyridi-nalmethylhydrazone and 2-pyridinal-dimethylhydrazone, those of the former being low spin and those of the latter high spin [49]. The temperature-dependence of the magnetism of the latter complex was not reported but may well be of interest. However, spin crossover [Fe(diimine)3]2+ systems have been characterised for systems where the incorporation of appropriate substituents has reduced the ligand field. 

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