Chemical reactivities of tetrapyrrole pigments: a comparison of experimentalbehaviour with the results of s.c.f.-pi-m.o. calculations.

Porphyrins are highly sigma-electron donating bases and very weak pi-acids. Hence they increase the electron density on central metal ions, e.g. iron, which leads to the specific reactivity of haem cytochromes, haemoglobin and oxidizing enzymes. The macrocyclic chlorin ligand behaves similarly but to a lesser degree which explains the comparably low oxidation potential of chlorophyll. Phlorins, oxophlorins, oxa- and aza-orphyrins, tetradehydrocorrins, corrins and biliverdins all produce metal complexes which have a similar geometry to that of metalloporphyrins, but their reactivity patterns are different. In contrast to the metalloporphyrins which undergo many fully reversible reactions, these compounds tend to irreversible addition and cleavage reactions. The tetrapyrrole ligands are stronger pi-acids than porphyrins. Results of some recent experimental work and pi-electron s.c.f. calculations are presented in support of these generalizations.