The Role of the Metal in Porphyrin Complexes 25 



electron donating groups cause the opposite effect (Brandt, Dwyer and 

 Gyarfas, 1954). 



Because of the non-equivalence of the electronic states in the oxidized and 

 reduced forms of many metal-porphyrins true equilibrium is not attained on 

 an electrode. This raises the question of the applicability of redox potential 

 results obtained from truly reversible model systems to the metal-porphyrins. 



The effect of substitution in the ligand molecule itself is but one aspect 

 of the problem. There is little precise information available from models on 

 the effect of the overall charge upon the redox potential, or of the effects 

 that might be anticipated from various ligands when added to a basic planar 

 complex. Recently, we (Dwyer and Goodwin, 1959) have prepared a large 

 number of mono- and Z?/5(bipyridine) and phenanthroline ruthenium(II) and 

 (III) complexes which serve as better models for the metal-porphyrin systems 

 than the //•/5(chelate) iron complexes. The Z?/5(chelate) complexes which 

 evidently are the more appropriate, however, always have the labile two groups 

 in the cis{\ : 2) position instead of the desirable trans{\ : 6) position. Ruthenium 

 is the heavier analogue of iron in Periodic Group 8, and unlike iron, the 

 mono- and bisichdaie) complexes do not disproportionate. The complexes 

 are spin-paired in both oxidation states and reversible redox potentials can be 

 obtained. By suitable replacement of the labile positions, anions, cations 

 and neutral complexes can be prepared. The effects of overall charge and of 

 the nature of the ligand are shown in Table 1 . 



Oxidation is greatly facilitated by lowering the positive charge. The replace- 

 ment of bipyridine, (pK^ = 4-33) by two molecules of the stronger base 

 pyridine (pA:„ = 5-20), also facilitates oxidation, but slightly. Large poten- 

 tial changes are associated with the co-ordination of ammonia, ethylene- 

 diamine and water. These seem much too large with the basic ligands to be 

 ascribed wholly to their greater strength as bases. Water, of course, is a 

 much weaker base than pyridine. The implied enhanced stability of the 

 oxidized state can be related in considerable part to the capacity of the 

 ligands to dissipate positive charge to their hydrogen atoms. The latter are 

 then more strongly solvated or can form hydrogen bonds to the solvent 

 water. At the acid concentrations used, dissociation of a proton from the 

 aquo groups is unlikely, though this would stabilize the oxidized form most 

 effectively by reducing the overall charge. 



There are still insufficient data to make much of a comparison between the 

 ruthenium systems and the metal-porphyrins containing various co-ordinated 

 addenda. The potentials of the latter systems certainly cover a much narrower 

 range, possibly because of the smaller overall charge. The replacement of 

 the water molecules (or water and hydroxyl) in protoporphyrin, for instance, 

 by pyridine only changes the potential from —0-14 V to -f 0-107 V. A much 

 more positive potential might have been anticipated. Similarly, the potential 

 of the dicyano-protoporphrin couple (—0-183 V) would be expected to be 



