64 



J. E. Falk and D. D. Perrin 



complexes fell into three classes, according to magnetic susceptibility, 

 spectroscopic and other properties : 



A 



Co++, Ni++ 



B 



Cu++, Ag++ 



C 



Zn++, Cd++ 



From the point of view of valence-bond theory, the conclusion was reached 

 that class A were spin-paired (3d4s4p^), class B spin-free (4s4pHd, 5s5p^5d 

 respectively) ; both A and B have great (qualitative) stability. Class C is also 

 spin-free {4s4pHd, 5s5p"5d) but of great (qualitative) instability. Within the 

 three classes the spectra were very similar, but from ^4 to 5 to C the visible 

 absorption moved to longer wavelengths. 



From Fig. 2, the essential difference between classes A, B and C is that 

 they have 0, 1 and 2 electrons, respectively, in the d^i_y2 antibonding orbital. 

 These electrons are favourably placed for strong repulsive electrostatic 

 interaction with electrons on the pyrrole-N's, so that the more electrons in 

 d^2_y2 the easier it is to displace 7T-electrons towards the periphery. Hence, in 

 agreement with experiment, the visible absorption maxima move to longer 

 wavelengths in passing from yl to jB to C. In the transition which gives rise 

 to the Soret band there is a displacement of some of the electron density to 

 the non-pyrrole (methene bridge) carbons, i.e. in a direction away from 

 d^2_y2 (Fig. 1). This transition should be facilitated in the same way and the 

 Soret maximum should shift to longer wavelengths in a similar sequence, as 

 observed by Falk and Nyholm (1958) for the protoporphyrin complexes in 

 benzene, which at the time appeared to be an inert solvent. It has recently 

 been found (J. N. Phillips, unpublished) that in fact this solvent modifies the 

 spectra of certain of the metalloporphyrins. The measurements have been 

 repeated in CCI4, which shows no evidence of interaction, and the follov/ing 

 maxima have been found : 



The a-bands have virtually identical maxima in benzene and in CCI4, as do 

 the Soret bands of the Co, Ni, Cu and Ag complexes. The Soret bands of the 

 Zn and Cd complexes in benzene, however, were at 415 and 423 m/< 

 respectively. 



In the square planar, low-spin, cobalt (d"^) and nickel (d^) complexes, since 

 there are the same number of electrons in d-^y (=2) and none in dj.2_y2, 

 similar Soret and visible maxima would be predicted and have been found. 

 In the same way the ferrous 6/>pyridine complexes which are diamagnetic 



