This work consists of three parts. The first part is a concise review on N-heterocyclic carbene (NHC) ligands and their complexes. In part II the experimental details were explained. In part III the synthesis, characterization and catalytic studies of 4,5-dihydro and disubstituted imidazolin derivatives were presented. Fundamental aim in design of the catalyst has been increased for the sake of the activity and the selectivity. Easy separation of catalyst from products and increased catalyst life is also main concern. Therefore, in order to synthesize the more efficient and the more stable catalysts than the present N-heterocyclic carbene precursors; LHX, containing functional/bulky groups that is placed on the nitrogen atoms and dihydro/substituted phenyl group at C4-C5 position of imidazolidin ring and the Rh(I)- and Pd(II)-NHC complexes from them have been prepared. The content of this book is based on an experimental study for my Ph.D. thesis submitted to Ege University, Izmir, Turkey in 2005.