|This Ph.D. thesis focuses on the synthesis and reactions of novel hafnacycles, e. g. hafnocene alkyne complexes or hafnacyclocumulenes. Spectroscopic and structural parameters as well as reactivity studies are used for comparison of these complexes with similar titanium and zirconium compounds. In some cases hafnocene compounds show an unexpected reactivity; for example in the synthesis of hafnocene alkyne complexes C-O, C-H and Si-C bond activations occur, which have not been observed to this extent for the lighter homologues. Moreover the results of this work show that the novel hafnocene alkyne complexes can provide a basis for the synthesis of catalytically relevant metallacycles. In addition, the synthesis of the first five-membered hafnycyclocumulene was achieved in this work as well as a first investigation of the reactivity of these compounds.