The Role of the Metal in Porphyrin Complexes 23 



oxidation of methaemoglobin removed an electron from peripheral carbon 

 atoms and not from the metal, which was taken as formally remaining in the 

 +3 state. 



Some sim.ple metal complexes containing nitric oxide provide examples of 

 where chemical, magnetic and electronic structure considerations fail to 

 establish the oxidation state of the metal. The ions [Fe(CN)5N0]-~ and 

 [RuCljNO]^" are both obtained by boiling salts of the tervalent metal ions 

 [Fe(CN)6]^~ and [RuCIgHaO]^^ with concentrated nitric acid. They are 

 diamagnetic and hence the oxidation state is assumed to be +2. It is proposed 

 that nitric oxide co-ordinates as N0+ following the loss of its odd electron 

 to the metal which is thereby reduced. A tt bond also is formed between a 

 d orbital of the metal and the vacant p orbital of the nitrogen. Exactly the 

 same ultimate electronic structure would result had the nitric oxide formed 

 the usual a bond, and the tt bond had come about by pairing the odd d 

 electron of the metal with the odd p electron of the nitrogen, or had the metal 

 lost an electron to nitrogen, which then utihzed four electrons to form a 

 double bond. It is questionable whether the donation of four electrons by 

 N0~ is more objectionable electronically than of two electrons by NO+. 

 The metals should then be considered in the +4 state, which is certainly more 

 consistent with the method of preparation and the resistance of the iron 

 complex to oxidation. 



Metal complexes are generally regarded simply as Lewis acid-base entities 

 but it is possibly more fruitful, especially in their oxidation-reduction 

 reactions, to regard some of them as integral internal redox systems in which 

 the metal alone is not the sole electron source or sink. Certain band spectra 

 of the strongly interacting transitional metal complexes with the porphyrins, 

 phenanthroline and bipyridine have been assigned to the transfer of negative 

 charge from the metal to the ligands, or in highly oxidized states of the 

 complex, in the opposite sense. In the similar activated states in which reac- 

 tion occurs we are, in effect, dealing with an oxidized or reduced ligand. 



Rapid racemization of both species has been found to occur when aqueous 

 solutions of d[Os bipy3]"'"+ and /[Os bipy3]+++ are mixed. This must proceed 

 through a peripheral electron, located most likely on a carbon atom in the 

 4-position to the nitrogen atom, leaking across to the oxidized form. Because 

 of the large organic molecules and the octahedral geometry, the metal atoms 

 themselves are inaccessible for direct electron transfer, even through a water 

 bridge (Dwyer and Gyarfas, 1952): 



J[Os bipy3]++ -^ ^[Os bipy3]+++ -f e 

 /[Os bipy3]+++ + Q-> /[Os bipy3]++ 



If we think of the oxidized complex as having an electron deficiency, i.e. a 

 positive charge, localized on a similar carbon atom, which is then solvated, a 

 water bridge is provided for electron transport (Fig. 5). 



