160 V. HEMATIN COMPOUNDS 



the rest of the molecule. This was confirmed by the x-ray studies of 

 Robertson {2281^, cf. also 175) on phthalocyanin (cf. Chapter III) 

 and nickel phthalocyanin. In metal-free phthalocyanin, the space 

 between the four central nitrogen atoms, normally occupied by two 

 hydrogen atoms, has a radius of 1.35 A, slightly larger than the 

 atomic (not ionic) radii of most metal atoms (nickel, 1.24; zinc, 1.32; 

 iron, 1.27; cobalt, 1.25 A). When nickel combines with phthalo- 

 cyanin, slight alterations of the angles in the inner sixteen-membered 

 ring, particularly on two of the four nitrogen atoms linking isoindole 

 rings, and slight alterations of the bond lengths, cause the distance 

 of the nitrogen atom from the center of the molecule to be decreased 

 from 1.92 to 1.83 A. This closely corresponds to the sum (1.85 A) 

 of the radii of doubly linked nitrogen (0.61 A) and nickel (1.24 A). 



The replacement of two hydrogen atoms, each bound to only two 

 of the nitrogens, by the nickel atom, equally bound to all four pyrrole 

 nitrogens, enables the whole molecule to become nearly tetragonal 

 in symmetry. In this way the four nitrogen atoms lie approximately 

 at the corners of a square whose center is the nickel atom. The 

 resonance of the molecule accounts for the fact that the three valencies 

 of the nitrogen atoms lie in one plane with an angle of 110° inside the 

 pyrrole ring and two angles of 125° outside. 



1.2. Bond Type in Metal Complexes 



It will be seen by reference to Chapter II, Section 6., that the 

 metal atoms can be bound in complexes of the type under discussion 

 by either ionic or covalent linkages, the number of unpaired electrons 

 in the 3d orbitals and hence the magnetic properties of the substance 

 differing according to the bond type. 



Our principal concern is with the hematin compounds themselves, 

 that is, those complexes in which the metal present is iron. The 

 simplest of these is heme, in which the ferrous iron atom is linked 

 only to the four nitrogens of the porphyrin. In this substance 

 magnetochemical data (2126,2127,2129,1173) show the existence of 

 four unpaired electrons, the bonds consequently being essentially 

 ionic. The fully ionic linkage would involve the replacement of the 

 two pyrrole hydrogens by the positively charged ferrous ion, the two 

 negative charges left on the porphyrin being equally distributed by 

 resonance among all four nitrogen atoms. Apart from any charges 

 resulting from ionization of side chain carboxyl groups, the complex 

 has a resultant charge of zero. 



