Complexes with Unsaturated Heterocyclic Ligands

Unsaturated heterocyclic ligands like pyrrols or porphyrins are formally derived from secondary organic amines and their metal derivatives are therefore amide complexes. The Ln-N r-bond lengths formed with simple monocyclic ligands 

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The use of )j2-benzyne and )j2-phosphabenzyne zirconocene in phosphorus chemistry is reviewed. The regiospecific reactions of these complexes with small unsaturated organic molecules incorporating phosphorus or not led to a variety of cyclic neutral or zwitterionic complexes showing a versatile behavior. New methodologies of synthesis of phosphorus heterocycles and of phosphorus ligands are described, as well as strategies of C-H and M-H (M=N, O, P, S) bond activations. 

While up to 1990 all attempts to isolate a stable N-heterocyclic carbene failed, metal complexes of unsaturated imidazol-2-ylidenes were known as early as 1968. The first complexes of this type were obtained by in situ deprotonation of imidazolium salts using mercury(ll) acetate or dimethylimidazolium hydridopentacarbonylchromate(-II) followed by coordination of the carbene to the metal center (Scheme 1.3). Shortly thereafter, the stabilization of the saturated imidazolin-2-ylidene in a metal complex was described by Lappert who treated electron-rich entetraamines of type 6=6 with coordinatively unsaturated transition metal complexes to obtain complexes with imidazolin-2-ylidene ligands (Scheme 1.3). ... 

Heterocyclic compounds such as pyrrole or thiophene can also act as 5- or 6-electron ligands to transition metals. Only a few of their complexes are known to date, but there seems no reason why a wide variety of unsaturated heterocyclics should not form stable, 7r-bonded metal complexes. Some examples, with preparative routes, are given below ... 

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