The Role of the Metal in Porphyrin Complexes 21 



groups. It is well known that the stability of metal complexes is usually 

 related directly to the strength of the ligand as a base, and this is merely 

 another way of expressing this concept. Electron withdrawing substituents 

 in the ligand molecule promote polarization in the wrong sense : compete with 

 the metal atom for electrons. These ideas have been expressed succinctly in 

 the "principle of essential neutrality" (Pauling, 1948). The pronounced 

 curariform activity of complex cations containing phenanthroline, and 

 bipyridine [M phengj^+j [M bipy3]++ in which the characteristic biological 

 response must be due to distributed charge, supports the principle (Dwyer 

 et al, 1957). 



The extrusion of protons during the formation of porphyrin metal com- 

 plexes reduces the charge by two units but, even in the reduced form, the 

 zero charge does not necessarily imply electrical neutrality of either the 



H 

 Oij — C- H.C- C C — CH, 



m-i .0 0, ,0 



NHj ~-o cr ^0 



1^1 II T 



'^'"'^*^'^^« HjC-C^^i— CH, 



° H 



Fig. 2 Fig. 3 



metal or the ligand. Apart from the electrical capacities of the substituent 

 groups, the transition metals are moderately electronegative. This property 

 may well be enhanced by the spin-paired electronic situation existing in strongly 

 interacting complexes. 



Recently, it has been shown that the methylene groups in the neutral 

 complexes Z?/.s(glycine)copper (Fig. 2) and rr/j'(glycine)cobalt are sufficiently 

 activated in this environment to enable Knoevenagel type condensations to 

 be performed with acetaldehyde (Sato, Okawa and Akabori, 1957; Ikutani, 

 Okuda, Sato and Akabori, 1959). A mixture of threonine and allothreonine 

 was obtained from the cobalt complex in the presence of sodium carbonate. 

 Djordjevic, Lewis and Nyholm (1959), found that nitrite ion and nitrogen 

 dioxide attacked the neutral complexes 6/Xacetylacetone)nickel (Fig. 3) and 

 /)/5(acetylacetone)copper, with the formation of complex organic nitrogen 

 compounds, as yet unidentified. It is probable that the sites of attack are the 

 activated resonating — CH — groups, which may carry a small positive 

 charge. 



In common with phthalocyanine, phenanthroline and bipyridine, metals are 

 bound more firmly in the porphyrins than might be anticipated from the base 

 strength of these ligands. The donor power of the ligand, concerned primarily 

 with the primary co-ordination or a bond, is responsible for the dissipation of 

 charge from the metal atom. It is believed that much of the bonding strength 

 of these molecules derives from at least two tt bonds in which the d electronic 

 orbitals of the metal overlap the vacant/? orbitals of the donor atoms. These 

 bonds tend to make the metal more positive. In the ferrocyanide ion 



