Spectra and Redox Potentials of Metalloporphyrins and Haemoproteins 65 



with an octahedral distribution of ^-orbital splittings, have no electrons in 

 d^2_yi and two in d^y. From a spectroscopic point of view they therefore 

 approach conditions for square planar nickel and cobalt complexes. Back 

 double bonding should shift the visible bands to slightly shorter wavelengths 

 and the Soret band to slightly longer wavelengths. Thus, for the proto- 

 porpiiyrin metal complexes the predicted order of the visible wavelength 

 maxima is: Co c;^ Ni > FePyg, and the observed values are 561, 561 and 

 558 m/< respectively. 



Although the stability of porphyrin metal complexes is due in large measure 

 to the difficulty of providing enough energy to rupture four bonds, whether 

 electrostatic or covalent, simultaneously, an additional effect in transition 

 elements is the ligand field stabilization energy. Considerations of L.F.S.E. 

 would suggest that the stability of these metalloporphyrins should lie in the 

 series, A> B> C. This is because electrons occupying low lying ^-orbitals 

 increase the stability of a complex, while those in higher (antibonding) orbitals 

 will remove some of this stability. The additional stabilization becomes zero 

 when all five of the J-orbitals are equally occupied. We estimate the L.F.S.E. 

 for the bivalent Co, Ni, Cu and Ag complexes of protoporphyrin dimethyl 

 ester to be at least 40 kcal, and this may be one factor contributing to the 

 quahtative differences in the difficulty of dissociation of the metal from these 

 complexes, as against the Zn, Cd and Pb complexes (which have no L.F.S.E.) 

 (Falk and Nyholm, 1958). For ferro- and ferrihaemoglobin the estimated 

 L.F.S.E.s are 20 kcal and zero, respectively. The great stability of ferric iron 

 in complexes is probably mainly due to the electrostatic forces of attraction 

 between opposite charges. 



The Effect ofLigands in the 5th and 6th Co-ordination Positions 



We restrict our discussion here to further complexes of iron-porphyrins, 

 i.e. the haemochromes. The 5th and 6th positions on these complexes, 

 above and below the plane of the haem molecule, correspond to positions 1 , 6 

 in octahedral complexes in general co-ordination chemistry. We discuss the 

 spectra of the Fe++ complexes only, since, as is commonly recognized, their 

 spectra are not complicated by the large component which has been attributed 

 (Williams, 1956) to charge-transfer from the ligand to the metal in the Fe+++ 

 complexes. 



Ligands to haem iron such as pyridine and chemically similar bases, and 

 CN~ ion, allow back-double-bonding of the electrons in the dy^ and d^^ 

 orbitals of the metal. The time these electrons spend in the plane of the 

 porphyrin molecule, and hence the average electron density in this plane are 

 reduced, so that it is harder for electrons to move towards the periphery, 

 and the visible absorption maxima move to shorter wavelengths. That is, 

 there is less electrostatic repulsion in the ground state if double bonding can 

 occur. 



