Spectra and Redox Potentials of Metalloporphyrins and Haemoproteins 71 



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DISCUSSION 



Correlations between Structure and Physical Properties 



George : With regard to the correlations that are being sought between cheniical reactivity, 

 physical properties and structural factors in co-ordination chemistry, and their exten- 

 sion to haemoprotein compounds, I would call attention to the more detailed and 

 revealing information that can often be obtained from A/f" and AS" data, which it 

 is not possible to get if only AG" data (e.g. E'q and pAT values) are considered. In 

 many cases, notably when ligand field effects are being investigated, A/f" is the 

 significant thermodynamic quantity: and the successful correlations based on AG" 

 probably result from the values of A^" remaining relatively constant throughout a 

 series of compounds. But in some instances apparently valid conclusions based on 

 AG" turn out to be rather misleading. 



For example there is the well-known correlation between the affinity of structurally 

 similar ligands for a metal ion and for the hydrogen ion; linear plots are obtained 

 for log (stability constant) against ^K. This suggests that the stronger the bond to 

 hydrogen, the stronger is the bond to the metal. Yet an examination of the rather scanty 

 thermodynamic data which are available indicates that the correlation is only determined 

 by the A//" values, which contain the bond energy terms, for certain families of ligands, 

 and that for others the AS" values dominate the relationship. 



Similarly oxidation-reduction potentials are often regarded as a relative msasure 

 simply of the energy required to remove an electron from the reduced form of the 

 couple. But this is not always so. For example, the Eq values for the Fe(dipy)3^+/^+, 

 Feaci^+/^+ and Fe(CN)6=^-/*- couples are about 1-0, 0-77 and 0-36 V respectively. Yet 

 this sequence is not determined by the electron-donating property of the ligands 

 following the sequence CN" < HjO < dipyridyl. The values of Ai/" for the cell 

 reaction in the presence of these ligands 



Fe'" + iH2^Fe" + H+ 



are about —30, —10 and —26 kcal/mole respectively, which show that the contri- 

 bution from the ionization potential of the Fe++ compounds is more nearly the same 

 for the dipyridyl and the cyanide complexes, and that entropy changes play a very 

 dominant role in determining the magnitude of Eq. This is not unexpected in view 

 of the entirely different charge changes in the cell reactions, -1-3 to -1-2 in contrast to 

 —3 to —4 respectively. Somewhat more surprising is the unfavourable entropy 

 contribution for the corresponding couples of haemoglobin and myoglobin as com- 

 pared to the favourable entropy contribution for the aquo-ion couple. The apparent 

 charge changes are -f-l to zero and -1-3 to +2 respectively, both of which should 

 make a favourable contribution to AS'". A//" is nearly the same as that for the aquo- 

 ion couple, and it is the entropy change which is responsible for the Eq values for the 

 two haemoproteins being some 0-6 V lower, i.e. at about 0-2 V compared to 0-77 V. 

 In setting up correlations, therefore, it can be extremely important to determine 

 whether A//° or A5" is the dominant factor. 



Spin Type and Spectra of Haem Compounds 



Williams: The spectra of metal porphyrins have recently been analysed by Gouterman 

 (/. chem. Phys. 30, 1 139, 1959). The suggestion that the band positions are solely due 



