80 HI. PORPHYRIN CHEMISTRY 



Tliere is one point which strongly supports the resonance formula (' with 

 hy<lrof;en bond linkage. The cations of the acid salts of pyrromethenes 

 (Fig. 17) can certainly he considered as resonance forms D\ and Di. If tiie 

 free ba.ses were tautomeric isomerides, whereas the salts were resonance 

 forms, one would expect that the additional resonance stabilization (diminu- 

 tion of energy) connected with salt formation would cause the pyrromethenes 

 to have a basicity comparable to that of the guanidines. The strong basicity 

 of guanidines and similar compounds is explained by Pauling {'212-'}, p. 213) 

 on the basis that the guanidinium ion is greatly stabilized by resonance 

 among three equivalent structures (.4,, Ai, and A3, Fig. 18), while the free 



-H 



Fig. 18. Resonance stabilization of guanidinium ions and free guanidine 

 (according to Pauling) . 



base itself resonates among three structures which are not equivalent, (^1, 

 lii, and //.i). the latter two contributing much less to the resonance. The pK 

 of pyrromethenes is not known exactly, but there can be no doubt that they 

 are rather weak bases. comiJaral)!e in strength to the porphyrins for which 

 a pK of 2. .5 has l)een found {Jt'TJf.). This can be accoimted for by the assump- 

 tion that the free base is already stabilized by resonance (formula (', Fig. Ki) 

 so that no great additional stabilization is caused by the formation of the ion. 



6.2. X-Ray Analysis of Phthalocyanins 



The most iinportaiit evidence in favor of resonance structure of the 

 porphyrins has, however, come from the studies of closely related 



