Electronic Structure and Electron Transport Properties of Metal Ions 1 5 



may be sened by drawing attention from time to time to the well-recognized approxi- 

 mations of a theory; an even more useful purpose is served by doing something 

 about them (see, for example, Hush and Pryce, J. chein. Phys. (1958), whose work 

 could well be extended to cover the transition from high-spin to low-spin states). 

 Williams: It is not suggested that A, the field strength in any one state, varies greatly 

 from every state to every other state. The variation of A will depend upon the character 

 of the different excited and ground states. The spectrochemical series was observed 

 to be a series roughly independent of whether one is dealing with low or high spin 

 complexes or with metals from different transition metal series. Will Orgel state 

 whether he believes this series to be also the series of the heats of interaction of ligands 

 with a given cation, independent of cation ? The case of the octahedral complexes 

 could be taken as an example. 

 Orgel has made this assumption himself in his discussion here (3a) and elsewhere. 



.o- o- 



Under 3 (b); if the NOg- group changes from ^N^' to -<-0— N^ then this is 



but an indication of a change in ligand character with cation or spin state and/or 

 valence state which I wish to demonstrate. This will be described shortly, not only 

 for this case, but for the SCN~ complexes also. Under 3 (c), the hydroxide ion pro- 

 duces spectroscopically the same effect in complexes of iron porphyrins where there 

 are no proteins. 



(4) needs amplification before I can discuss it. I agree with Orgel's conclusion. My 

 criticisms stand if the over-simplifications of theory lead to inconsistency with experi- 

 ment. I say that they do. 



Spin-states of Haem Compounds 



Falk: I agree with Orgel that the relatively easy transition of many haemoproteins 

 between the low- and high-spin states is very interesting. This phenomenon has 

 fascinated me for some years, and Falk & Nyholm {Current Trends in Heterocyclic 

 Chemistry (1958), p. 130) have discussed it briefly. But I think it is pertinent to 

 remark that this phenomenon is established only for compounds of the haemoglobin, 

 catalase and peroxidase types, and not for haemoproteins which are electron-transport 

 agents in the classical cytochrome c fashion. I am not aware of any evidence, from 

 magnetic susceptibility m.easurements, of high-spin low-spin changes in cytochromes 

 of c, b or a types. If I may be allowed to guess, I would suggest that of these cyto- 

 chromes, the properties of cytochromes a point to them as the most likely of the three 

 types to shov/ this phenomenon. 



Orgel mentioned at one point the old observation of three unpaired electrons in 

 ferric phthalocyanine chloride. In this context I think it is interesting to draw attention 

 to the unpublished investigations of Nyholm and myself, mentioned in Falk and 

 PeiTin (this volume, p. 56) on ferriprotoporphyrin chloride ("haemin chloride"). 

 We found no conductivity in nitrobenzene solutions, indicating that the compound 

 is not an electrolyte, and in view of the 5 unpaired spins, reported in the literature 

 and confirmed by us, have suggested that it must be a square pyramidal complex 

 with AsApHd'^ hybridization. 



Orgel: The observations of Falk and Nyholm are very relevant here. I v/onder whether 

 the ferric protoporphyrin chloride has the same structure both in the solid and in 

 nitrobenzene solution. Perhaps it would have the 3-spin ground state in nitrobenzene 

 corresponding to a pyramidal structure, but have five unpaired electrons and an 

 octahedral structure (with shared chloride ions) in the solid. 



George: I think there is some doubt whether the paramagnetic resonance absorption 

 measurements at pH 7 and 8-5 carried out by Gibson, Ingram and Schonland {Disc. 

 Faraday Sac, 26, 72 (1958)) prove that ferrihaemoglobin hydroxide is a mixture of high- 

 and low-spin forms. First, the pK of the ionization is about 8 at room temperature 

 so that at pH 7 only about 10% of the ferrihaemoglobin would be present as the 

 hydroxide. Secondly, Keilin and Hartree {Nature Land., 164, 254 (1949)) have shown 

 that on cooling the conjugate acid is favoured in the dissociation equilibrium, as 



