SOME PHYSICAL PROPERTIES AND CHEMICAL 

 REACTIONS OF IRON COMPLEXES 



By R. J. P. Williams 

 Inorganic Chemistry Laboratory and Wadham College, Oxford 



This article is intentionally speculative in that it will use a comparison 

 between known properties and reactions of some simple iron complexes with 

 chelating and conjugated ligands on the one hand, and of the wide range of 

 haem-containing compounds of biology on the other, in order to make 

 comments about the structure and mechanism of reactions of the latter 

 molecules. First we restrict our attention to simple iron complexes. There 

 are three classes of these. In class I all the properties and reactions are 

 consistent with the iron (Fe++ or Fe+++) being in a high-spin state (ionic 

 complexes). In class III the properties are consistent with the cation (Fe++ 

 or Fe+++) being in a low-spin state (covalent complexes). In the intermediate 

 group, class II, the properties and reactions are consistent with the cation 

 being present in the complexes as an equilibrium mixture of high- and low- 

 spin states. Table 1, which hsts the properties and reactions of the groups, 

 is a summary of evidence presented in detail elsewhere (Williams, 1955, 1956, 

 1958, 1959). The change in the character of the iron complexes (from one 

 group to another) is to be associated with the change in the strength of the 

 effective ligand field increasing from class I through class II to class III. The 

 porphyrin unit supphes a ligand field which, together with groups above and 

 below the porphyrin plane, places its iron complexes in either class II or 

 class III, although the extreme weak-field members in class II can have 

 properties very like complexes in class I. The evidence for this statement is 

 collected and discussed elsewhere (Williams, 1955-59). Here we take this 

 position as estabUshed. However we will illustrate it by reference to some 

 physical properties of porphyrins. 



SPECTRA 



The absorption bands in the spectra of iron porphyrins have been discussed 

 (Williams, 1956) except for the Soret band. Here we will examine the shifts 

 in the position of this band ignoring the others. Some band positions are 

 given in Table 2. In Table 2 wavelengths are in m^. The description 'ionic' 

 and 'covalent' is only applicable to the first three rows of data. 



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