pi-Metal complexes of tetrapyrrolic systems. A novel coordination mode in porphyrin-like'' chemistry

The coordination chemistry of porphyrins and related tetrapyrrolic ligands has traditionally centered around the ability of these systems to form pyrrole N-ligated complexes via the formation of sigma bonds, either within the N-4 core or displaced above it. In fact, such sigma-complexes are known with almost every metal cation in the periodic table. However, a growing number of pi-complexes derived from tetrapyrrolic ligands have been reported in recent years. The underlying coordination mode, while still novel in the context of porphyrin-like'' chemistry, is already being recognized for the effects it can impart over the reactivity, as well as the spectroscopic, redox, electronic, and optical properties of various oligopyrrolic macrocycles. This critical review summarizes accomplishments made in this fast-emerging field (59 references).