Ferrihaemoprotein Hydroxides 1 07 



Kotani, 1949). According to the then current theory, Coryell et ah (1937) 

 described the bonding of the iron as essentially ionic in the first group, and 

 essentially covalent in the second group through M-AsAp^ orbital hybridization. 



It was noted that the intermediate value for the hydroxide corresponded 

 more nearly to the theoretical value for the spin contribution of three unpaired 

 electrons, which would be anticipated for a square planar ferric complex with 

 essentially covalent bonding through lidAsAp'^ orbital hybridization. But since 

 the chemical structure of ferrihaemoprotein complexes requires octahedral 

 co-ordination, it was suggested that while the moment of the hydroxide 

 results from the electronic structure with three unpaired electrons, the four 

 covalent bonds resonate among the six co-ordinated groups. 



This interpretation was widely accepted, and was not seriously questioned 

 for many years. In an extensive review of co-ordination compounds, Taube 

 (1952) proposed that the utilization of ^-orbitals with the next higher principal 

 quantum number might occur in complexes having the high magnetic 

 moments. For example, the bonding in the cyanide complex of ferrihaemo- 

 globin would be attributed to MHsAp^ hybridization as before, but in the 

 fluoride complex to AdHsAp^ hybridization, leaving the five unpaired electrons 

 in the 3^-orbitals unaffected. Taube commented that with certain metal ions a 

 close balance might occur between the energies of these inner and outer 

 orbital complexes, so that with some ligands the complexes would have high 

 magnetic moments, and with others low magnetic moments, e.g. KgCoFg, 

 which is paramagnetic in contrast to the cobaltic amine complexes which are 

 diamagnetic. Coryell, Stitt and Pauling's measurements on haemoglobin 

 derivatives showed that Fe++ and Fe+++ porphyrin compounds come into the 

 same category; and Taube went on to suggest that, since 0H~ is intermediate 

 in polarizability between HgO and SH~, an alternative explanation for the 

 anomalous magnetic moment of the hydroxide is the presence of inner and 

 outer orbital complexes in equilibrium. 



During the last seven years a more detailed understanding of the electronic 

 structure of transition metal compounds has been arrived at using that com- 

 bination of the molecular orbital method and the simple rigid crystal field 

 method which has come to be known as ligand field theory (see Griffith and 

 Orgel, 1957). Ligand field theory is in many ways more general than Pauling 

 and Taube's schemes which really only discussed directed bond orbitals on 

 the central ion formed from atomic orbitals which were assumed to be 

 unchanged from those in the free ion. On the other hand, because of its 

 close relationship to the simple crystal field model, it preserves to a consider- 

 able extent the possibility of making detailed interpretations of the electronic 

 structure and semi-quantitative calculations of experimental quantities. 



We use here for convenience, but not necessity, the language of the crystal 

 field theory. Briefly then, for the case of a regular octahedral complex, the 

 five orbitals of the 6?-shell, which have the same energy in the free ion, are 



H.E. — VOL. I — J 



