Spectra and Redox Potentials of Metalloporphyrins and Haemoproteins 73 



Soret band data are not available; magnetic susceptibility measurements have not 

 yet been made on these. Because I suspect that there may be surprises in store in the 

 magnetic properties of some of these new compounds, I suggest that we should be 

 careful to refer to low-spin and high-spin compounds only when definitive magnetic 

 susceptibility measurements have been made. If inferences are being made from 

 spectra only, pending magnetic measurements, perhaps we could say 'low-spin spectral 

 type', and so on. With this reservation, I do agree with Williams that we must continue 

 to develop correlations between spectrum and spin-type. 

 Perrin : I suggest that the electronegativity of the central metal is of less consequence in 

 determining spectral shifts in porphyrin complexes than are the number of its d- 

 electrons and the directions of the orbitals they occupy. Falk and I discussed this. In 

 the case of ferrous ion the change from a high-spin to a low-spin configuration means 

 that the unpaired electrons in the d^^_y''- and d^^ antibonding orbitals vacate these 

 positions and all six of the rf-electrons fill, in pairs, the d^y, dy^ and d^.^ orbitals. From 

 the spatial distribution of these orbitals it is easy to see that this leads to an increase 

 in electron density along the xy axes (i.e. through opposite methene carbon atoms) 

 and a reduction in density along the x and y axes; the net increase in electron density 

 in the plane of the porphyrin ring is probably small. If the transitions giving rise to 

 porphyrin spectra are of the types shown in Fig. 1 of our paper (based on Longuet- 

 Higgins, Rector and Piatt, 1950; Seely, 1957; and Kuhn, 1959; see Falk and Perrin, 

 p. 56, for references), what effects have the change from a high-spin to a low-spin 

 ferrous complex on these transitions? The important thing to remember, and what 

 distinguishes low-spin ferrous complexes from low-spin ferric complexes, is that in 

 the former the dx^_y''- electron of the high-spin state has gone into the d^y orbital, 

 whereas in the latter it is in the dy, or d^^ orbital. In ferrous complexes this makes 

 the Soret transition more difficult because in the excited state there is increased electro- 

 static repulsion with electrons on the methene carbon atoms. This is diminished by 

 back-double-bonding whh ligands in the 5th and 6th co-ordination positions and 

 this might be expected to displace Amax to longer wavelengths. On the other hand, 

 for the visible bands in low-spin ferrous complexes there is decreased electrostatic 

 repulsion in the excited state but this effect is reduced by back-double-bonding, so 

 here Amax moves to shorter wavelengths the greater the back-double-bonding. 



If the electronic transition which gives rise to the Soret band does not involve a 

 net movement of electronic charge away from the metal in the high-spin ferric and 

 ferrous complexes (which have symmetrical d electron distributions in the plane of 

 the porphyrin) Amax would be expected to be at shorter wavelengths for ferric than 

 for ferrous, irrespective of whether electron density on the pyrrole nitrogens is decreased 

 as suggested by Williams or increased as suggested by Falk and myself. 



In the ferric haemoproteins the Soret band shifts to longer wavelengths in the low- 

 spin complexes. This is in line with expectation. The change from high-spin to 

 low-spin includes taking an electron from d^.-^^y"- and putting it in dy,, d^^ (or possibly 

 dxy): this makes it easier to put more 7r-electron density on the pyrrole-nitrogens in 

 the excited state and hence lowers the energy needed for the transition if it is of the 

 form Falk and I suggest in our Fig. 1 . The opposing effect of increasing the electrons 

 in d^y, dy, or d,j. would be diminished where back-double-bonding is possible and, in 

 fact, one might expect, other things being equal, that the longest wavelengths for Soret 

 maxima will be given by the best back-double-bonding ligands. 

 Williams : As far as I can see Falk and Perrin used the same theory as I do, that of Piatt, 

 in the discussion of the spectra of porphyrins (see Chem. Rev. 56, 299, 1956). There is 

 then the question of how metals affect the spectra. I appear to agree with the treat- 

 ment given recently by Gouterman (J. chem. Phys. 30, 1139, 1959), and which is the 

 best I know, while Falk and Perrin's discussion would say something different. I say 

 that the Soret band shifts to longer wavelengths with decrease of effective electro- 

 negativity of the central metal. From what Perrin says I feel he must conclude that 

 the opposite is true. 

 Lemberg : I am not convinced that there is a general relationship between the Soret band 

 positions and the spin type. The Soret band of high-spin ferrohaemoglobin lies at 



