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Investigations of ruthenium halfsandwich complexes have mostly focused on Cp and Cp* based derivatives due to the availability of easily accessible starting materials. However, it is interesting to have complexes with substituted cyclopentadienyl ligands in order to study the influence of the π-ligand on the chemical behavior of the complexes, as well as to fine-tune their properties. During the course of this thesis, ruthenium complexes with novel, highly substituted cyclopentadienyl ligands ('Cp^') were prepared in one-pot syntheses from commercially available starting materials and under mild reaction conditions. The complexes are versatile starting materials leading to a variety of Cp^Ru complexes through facile ligand exchange reactions. The chemistry of Cp^Ru-complexes has been investigated and the general reactivity pattern was found to be similar to that of CpRu and Cp*Ru complexes. However, in several instances it was observed that the steric and electronic properties of the Cp^ ligand have a strong impact on the properties and chemical behavior of the complexes. As a consequence of the sterically demanding Cp^ ligand, coordinatively unsaturated complexes, that are otherwise inaccessible, have been stabilized. In addition, the presence of the Cp^ ligand resulted in unusual CO insertion reactions and in an unprecedented rearrangement of cyclooctadiene. Some Cp^Ru-complexes turned out to be very efficient catalysts for organic transformations such as the [2+2+2] alkyne cyclotrimerization and atom transfer radical cyclizations. Activation of molecular oxygen was also accomplished with Cp^Ru complexes. In vitro studies with Cp^Ru complexes have shown that they exhibit high cytotoxicity towards human cancer cell lines.