BOND TYPE rX METAL COMPLEXES 161 



Hemin and hematiii resemble heme in having essentially ionic 

 bonds, but in this case, the iron being in the ferric state, the complex 

 has a unit residual positive charge. It is in consequence associated 

 with a negatively charged dissociable ion, for which in hematin, 

 when the ion is hydroxyl, a definite dissociation constant can be 

 determined. 



There is still some disagreement as to the type of bonds in heme, 

 hemin, and hematin. Pauling believes the bonds to be of mixed 

 ionic-covalent character, involving one 4^ and three 4/> orbitals, with 

 the 3d orbitals filled with unpaired electrons. Huggins {1362) assumes 

 a covalent linkage through the 4rf orbitals. Such porphyrin metal 

 compounds do not differ much in their chemical properties (solubility 

 in organic solvents, firmness of linkage between metal and porphyrin) 

 from the covalently linked compounds, either because the metal ion 

 still resides in the center of the molecule and its electric charge is 

 thus screened off, or because the linkage is also essentially covalent 

 though not of d'^sp^ type. 



When additional molecules enter the complex, the bond type of 

 the iron may be either ionic or covalent according to the circum- 

 stances. The introduction of two simple nitrogenous molecules into 

 a ferroporphyrin (heme) gives rise to a substance known as a hemo- 

 chrome ("hemochromogen") (c/. Section 2.), which is diamagnetic, 

 and consequently has no unpaired electrons. The bond type is thus 

 d^sjp^ covalent, four of the octahedrally disposed bonds being coplanar, 

 and directed to the porphyrin nitrogen atoms, the fifth and sixth 

 being at right angles to the plane of the porphyrin and, respectively, 

 above and below it. These latter two bonds are utilized in attaching 

 the two additional nitrogenous molecules. We shall have occasion 

 in subsequent sections to discuss the type of linkage formed by the 

 iron in a variety of individual cases, particularly where more complex 

 coordinating molecules such as proteins are involved. 



Magnetochemical measurements show that nickel phthalocyanin 

 and nickel porphyrins are diamagnetic, having no unpaired electrons 

 (117^,1545,1546). The bonds are hence covalent, and as shown in 

 Figure 2 (Chap. II, Sect. 6.1.), are of dsp- type. Since dsp~ bonds 

 are directed toward the corners of a square, combination of nickel 

 with phthalocyanin and porphyrins in the planar arrangement offers 

 no difficulties. However, other metals, such as magnesium and 

 beryllium, which usually have a tetrahedral sp^ symmetry of their 

 bonds, combine with these substances giving complexes which appear 



