180 



V. HEMATIN COMPOUNDS 



— which is unhkely in view of the effect of pli on spectrum and 

 stabiUty — Clark's postuhite requires the assumption of hepta 

 coordination, and the fornmla III of Figure 5. 



N 



B 



(I) 



B OH 



N I X 



B 



(III) 



Fig. 5. Structure of hem/chromes. 



This formuhi is difficult to reconcile with the sterical configuration 

 of the hematin molecule. Since the bonds are essentially covalent 

 any suggestion that the iron atom lies outside the plane of the 

 porphyrin nitrogens is not acceptable.* 



Hepta coordination lias been found for some compounds, e.g., (CbFT)^" 

 and (TaF;)^" {rf. Jl::!/>, p. 110), hut in thee tlie bond cliaracter is predomi- 

 nantly ionic. Nevertlieless a sterical arrangement similar to that in these 



Fig. 6. Stereochemistry of hepta coordination. 



complex fluorides, with four porphyrin nitrogens at the corners of one square 

 face of a trigonal prism, the hydroxyl grouj) and one pyridine nitrogen at 

 the other corners of this prism, and the second pyridine nitrogen al)ove the 

 center of tlie opposite side of the first-mentioned scjuare face, must be con- 

 sidered as a rather improbable possibility (Fig. 6). 



* The suggestion of Michaelis (l'.)-i6) thiit Hiikage to hydroxyl may replace honds 

 hetween iron and porj)hyrin nitrogen caimot l)e accejjted. Whatever happens to the 

 bond, the liy<lro.\yl grouj) cannot enter tlie [xjsition j)reviously occupied by the por- 

 phyrin nitrogen, since tiie rigid arrangement of tlie porphyrin molecule keeps the iron 

 atom in its i)Iace. « 



