162 V. HEMATIN COMPOUNDS 



from spectroscopic evidence (see Section 1.3.) to contain covalently 

 linked metal. It would seem that the steric requirements of the 

 porphyrin impose upon these metals the necessity of forming a type 

 of covalent bond different from that normal to them, having coplanar 

 directional character.* Alternatively it is possible that, despite the 

 spectrum, the bonds may be ionic, in which case the difficulty 

 vanishes. 



1.3. Absorption Spectrum and Bond Type 



It will be seen later that hematin compounds tend to show certain 

 group similarities of spectral type. Although present knowledge of 

 the relationship of spectrum to structure is inadequate, it is possible 

 to discern some reasonably well defined relationships between the 

 type of spectrum and the nature of the linkage of the metal atom. 



Theorell {2775) distinguishes four types of absorption spectra of 

 iron porphyrin compounds in a scheme which is given here in a 

 somewhat modified form. 



1. Ferric compounds with essentially ionic bonds. Brown or green, with 

 absorption bands in the red. Examples: hemin, hematin; hemoglobin; hemi- 

 globin fluoride; peroxidase, peroxidase fluoride; catalase. catalase azide. 



2. Ferric compounds with essentially covalent bonds. Red compounds 

 with one broad (cyanide compounds) or two narrower rather flat bands in 

 the green. Examples: hemoglobin cyanide and hydrosulfide; hemichromes; 

 ferricytochrome c; peroxidase cyanide and hydrosulfide; alkaline peroxidase; 

 catalase cyanide. 



* For the metal complex salts of some pyrromethenes one would expect tetrahedral 

 arrangement of the metal valencies, since planar arrangement would appear to cause 

 a considerable distortion by overlapping of side chains, e.g., methyl groups in the 

 a-positions {2172): 



Indeed, the nickel complex has been found to be paramagnetic {1906) ; the palladium 

 complex is diamagnetic, but evidence from this as to the type of bondage is inconclusive. 



