The reaction is in large parts dependent on the availability of suitable catalysts. They need to show a high activity and efficiency as well as a great stability. To meet these requirements, substantial improvements of the ligand sphere of the organometallic catalysts have been made during the thesis. The first chapter of the thesis shows new ways to immobilised olefin metathesis catalysts. The introduction of N-heterocyclic carbenes as replacement for the hitherto used phosphines in ruthenium alkylidene complexes has led to the development of permanently immobilised catalysts that can easily be filtered off after the reaction has taken place.

In a number of reactions no further purification of the product was necessary and the recovered catalyst could be reused up to four times in subsequent reactions. The second chapter deals with the design, synthesis and characterisation of entirely phosphine-free homogeneous ruthenium alkylidene catalysts. They possess an unprecedented stability against moisture and air and show a high activity and selectivity in olefin cross metathesis. The last chapter describes a new way for the introduction of chirality into N-heterocyclic carbene ligands. As a proof of concept, ruthenium alkylidene complexes bearing such chiral moieties have been synthesised, characterised and used in olefin metathesis reactions.