60 



J. E. Falk and D. D. Perrin 



of the pyrrole rings. The ^'^-orbital is one of a pair of equal energy strongly 

 polarized along and perpendicular to the axis of the two NH's. Although the 

 overall movement of electronic charge towards the periphery is negligible, 

 the increase of electron density on the non-pyrrole carbons and a pair of 



Fig. 1. Nature of transitions giving rise to porphyrin spectra. The signs + 

 and — indicate wave functions. The areas of the circles give electron densities 

 in the vicinities of atoms ; the atomic distances are the same as in phthalocyanine 



(Robertson, 1936). 



opposite nitrogens (Fig. 1) affects the absorption spectrum of porphyrins 

 and metalloporphyrins. 



Co-ordination of Porphyrins with Divalent Metals 



The property that distinguishes transition metals from other elements in 

 the Periodic Table is that their 6?-orbitals are incompletely filled with electrons. 

 These orbitals have the directions in space shown in Orgel's paper {loc. 

 cit.). Depending on whether electrons occupy these orbitals singly or in 

 pairs, complexes will be para- or dia-magnetic. 



This difference in magnetic properties is interpreted in the Valence Bond 

 Theory as distinguishing two types of complexes. The main concept underlying 



