In these days a renaissance of metallacycles as an increasingly important class of organometallic compounds for synthetic and catalytic applications is evident, making such very attractive for a plethora of investigations. Titanocene and zirconocene bis(trimethylsilyl)acetylene complexes, regarded as three-membered metallacycles (1-metallacyclopropenes), present a rich chemistry towards unsaturated molecules. By elimination of the alkyne these complexes form by reaction with unsaturated compounds five-membered titana- and zirconacycles, all of which are relevant to stoichiometric and catalytic C–C coupling and cleavage reactions of unsaturated molecules.

This perspective gives an overview of recent developments in the chemistry of heterometallacycles of group 4 metallocenes. This class of compounds has been known for several decades now, however, in recent years significant efforts were made in order to further explore the structural scope of these complexes and their potential in catalytic applications. This overview shows some special heterometallacycles which could be of great interest for theoretical and synthetic chemists, owing to their unusual structural and bonding parameters as well as their applicability in a wide range of synthetic and catalytic transformations.