Spectra and Redox Potentials of Metalloporphyrins and Haemoproteins 63 



otherwise there will be as little spin pairing as possible. For example, in 

 haemoglobin the situation is probably something like that shown at A in 

 Fig. 2. The diflference in ligand field stabilization energy (L.F.S.E.) between 

 the low-spin and the high-spin alternatives is not great enough to prevent the 

 ^/-electrons of the ferrous iron from spreading over all five of the 3fif-orbitals, 

 four of which are occupied singly while the fifth, and lowest, holds a pair of 

 electrons. Replacement of the water molecule in the sixth co-ordination 

 position by oxygen, to give oxyhaemoglobin, displaces conditions towards B 

 in Fig. 2, where the L.F.S.E. difference is sufficient to make the filling of the 

 three lowest orbitals with pairs of electrons the more energetically-favoured 

 process, giving a diamagnetic complex. For theoretical reasons complexes 

 intermediate between high-spin and low-spin are unlikely. It is unnecessary 

 to postulate any sudden changes in the nature of the ligand-metal bonds and, 

 in fact, it is an important consequence of this theory that the magnetic 

 behaviour of complexes does not provide a means of classifying complexes 

 into 'outer-' and 'inner-orbital' or 'ionic' and 'covalent'. 



THE SPECTRA OF SOME PORPHYRINS AND THEIR 

 METAL COMPLEXES 

 Because the visible absorption spectra of porphyrins are associated with a 

 displacement of electrons towards the periphery of the porphyrin nucleus, 

 any effect which results in an extension of the distance the electrons can move 

 in this direction reduces the energy required for these transitions, so that 

 visible absorption maxima move to longer wavelengths. Any effect operating 

 in the opposite direction moves these absorption maxima to shorter wave- 

 lengths. A number of examples illustrate this. 



The Ejfect of Porphyrin Side-chains 



As has already been seen (Table 2) the spectra of a series of free porphyrins, 

 and of the pyridine haemochromes of their Fe '^ "^ complexes, move to longer 

 wavelengths stepwise as the electron-attracting power of the porphyrin side- 

 chains increases. This porphyrin side-chain effect operates similarly in the 

 simple (square) porphyrin complexes with a variety of divalent metals (cf. 

 Table 2 of Falk and Nyholm, 1958), and indeed throughout the metallo- 

 porphyrins of all types, including, in a broad sense, the haemoproteins (cf. 

 Table 1). Among the latter, replacement of the protein with pyridine is a 

 convenient way to obviate effects on spectrum peculiar to the protein; 

 pyridine haemochrome spectra reflect accurately the effects of electron- 

 attracting side-chains on the haem nucleus. 



The Effect of Co-ordinated Metal Ions 



Falk and Nyholm (1958) have compared the protoporphyrin complexes of 

 a number of different divalent metal ions. It was found that the following 



