Ruthenium complexes are well investigated as molecular catalysts in synthetic chemistry to clarify their great potential for a wide range of organic reactions, such as reduction, oxidation, and carbon-carbon bond formations. As their reactivities in various catalysis strongly depend on their auxiliaries, for example, phosphines, nitrogen ligands, carbonyl, or halogens, design and selection of the auxiliaries suitable for an objective reaction become very important for the discovery of a new catalyst. It is the catalysts for the cyclopropanation of olefins with diazoesters.