20 F. P. DWYER 



The two co-ordination positions at right angles to the plane of the quad- 

 ridentate ring can be occupied by a variety of ligands: water, halide or 

 cyanide ion, organic bases, the histidine anion, carbon monoxide. There is a 

 good deal of still somewhat empirical evidence, e.g. labihty, that these out of 

 plane bonds are rather long, and the geometry is therefore tetragonal. The 

 bonds may be so long that the complex is essentially planar. Long bonds in 

 the polar, (1:6), positions of many copper, nickel and palladium complexes 

 are well known (Nyholm, 1953; Nyholm et al, 1956). In cytochrome c the 

 interaction of the imidazole groups is sufficient to promote the maximum 

 electron pairing in both oxidation states and the geometry must be octahedral. 



The higher oxidation states favour co-ordination of anions because of the 

 greater polarizing power of the metal and also the greater electronegativity. 

 In the oxidized forms of the metal-porphyrins there is thus a stronger tendency 

 to co-ordinate OH' and CI' to available sites or by displacement of another 

 ligand. The ionic structure ascribed usually to haemin chloride is unlikely. 



The 'Vra«5'-effect" may be of considerable significance in the 6-co-ordinate 

 metal-porphyrins. The effect, which has been extensively studied in planar 

 complexes (Chatt et al, 1955), refers to the labilizing effect of groups, e.g. 

 CI, CN, CO, on other groups or ligands attached in the opposite {trans) 

 position. In octahedral complexes, though the chemistry is more compli- 

 cated, the "/ra/75-eflFect" has been fruitful in elucidating substitution reaction 

 mechanisms (Quagliano and Schubert, 1952; Basolo and Pearson, 1958). 

 Strongly trans influencing groups : CO, CN, or the thiol anion, should modify 

 the strength of attachment or even the properties of other ligands in the polar 

 position. 



The imposition of a fixed spatial arrangement on groups attached in the 

 1 : 6 co-ordination positions is an important function of the metal atom, 

 especially when it is realized that donor atoms of the protein itself are often 

 linked in this way. Part of the functional role of the cobalt atom in vitamin 

 Bi2 is the rigid and unique conformation imposed on the large organic 

 moiety. Another part is probably the lability of the sixth co-ordination 

 position normally occupied by cyanide ion, water or hydroxyl, but which can 

 be used to attach a donor atom from protein. 



CHARGE DISTRIBUTION IN COMPLEXES 

 The fundamental principles involved in the formation of metal complexes, 

 first enunciated by Pauling (1938) have been elaborated by numerous authors 

 (Martell and Calvin, 1952; Basolo and Pearson, 1958). Co-ordination of a 

 Hgand to a metal ion decreases the charge on the ion and makes the donor 

 atom more positive. Since donor atoms are amongst the most electronegative 

 of the elements, part or most of the positive charge spreads over the ligand 

 molecule. In effect, this means that the ligand molecule is polarized, with the 

 withdrawal of peripheral electrons, or electrons from electron donating 



