Spectra and Redox Potentials of Metalloporphyrins and Haemoproteins 61 



this theory, as applied to co-ordination complexes, is that suitable vacant 

 orbitals of the metal are hybridized, and these hybrid orbitals are filled by 

 electron pairs 'donated' by ligand atoms with the formation of cr-bonds. To 

 be suitable for hybridization in this way an orbital must have an appreciable 

 component in the directions finally occupied by some or all of the ligands. 

 The diamagnetism of pyridine haemochromes is interpreted to mean that 

 two of the 3i/-orbitals of Fe++ {d^i^yi and d^^ are used in this hybridization 

 and are occupied by two pairs of ligand electrons ; the electrons already in 

 these orbitals are forced to pair up in the remaining 3c/-orbitals. Such com- 

 plexes have long been called 'covalent' and, more recently, 'inner-orbital', 

 'spin-paired', or 'low-spin'. Their formation is favoured by ligands of low 

 electronegativity and in octahedral complexes such as pyridine haemochrome 

 they are described as being ZdHsAp^, or d'^sp'^, types. 



On the other hand, haemin chloride, like the Fe+++ ion itself, has 5 unpaired 

 electrons. Such paramagnetic complexes ('semi-ionic', 'outer-orbital', 'spin- 

 free', 'high-spin') are generally formed by ligands of high electronegativity 

 which, in addition, have little or no d acceptor capacity for double bonding 

 (e.g. F" as against pyridine-N). It is now believed that high-spin complexes 

 do have some degree of covalent bonding (cf. Craig et al., 1954) and it is 

 convenient to regard haemin chloride, for example, as a hybrid of the type 

 AsApHd"^. There is little doubt that in 'haemin chloride' (ferriprotohaem 

 chloride), traditionally regarded as a square-planar complex with the Cl~ 

 ionically associated, the Cl~ is 'co-ordinately' bound. Falk and Nyholm 

 (unpublished) have found a 0-001 m solution in nitrobenzene to be a non- 

 conductor of electricity. Under these conditions, univalent electrolytes have 

 conductivities of 20-30 r.o. It appears likely that ferriprotohaem hydroxide 

 ('haematin') is a similar complex. 



A more recent and more satisfying interpretation of the magnetic and other 

 properties of complexes is provided by the Ligand Field Theory (Griffith and 

 Orgel, 1957). In essence, this theory says that as co-ordinating groups, or 

 ligands, approach a metal ion to form a complex, ^-orbitals pointing towards 

 the ligands are raised in energy and electrons in them become less stable, 

 while J-orbitals pointing away from the ligands become more stable. Bonding 

 molecular orbitals are formed by suitable electron-filled orbitals on the 

 ligands with the metal's vacant s- and /7-orbitals and the ^/-orbitals which 

 point tov/ards the ligands; in octahedral complexes, the c^-orbitals involved 

 are d^2_y2 and d^2. Any electrons already in these ^/-orbitals are removed by 

 promoting them into antibonding orbitals, but their presence reduces the 

 stability of the final complex. In any complex the magnitude of the differences 

 in the energy levels of the various J-orbitals is a function both of the ligand 

 and of the geometrical shape of the complex itself. 



The electrostatic effect of ligands in splitting these levels is enhanced by 

 *back double bonding', which arises from the ability of suitably placed, 



