This volume starts the updating of the information on organonickel chemistry, one of the most active areas of modern organometallic chemistry. It covers all mononuclear compounds containing one nickel-to-carbon bond. The main part is devoted to the family of planar NiII compounds, RNi(D)2X, which within 20 years has increased from about 250 to more than 1000 members. This research interest has been stimulated by the fact that many compounds insert various unsaturated species into the Ni-C bond to create new organonickel species. Moreover, these compounds are important as intermediates in many catalytic processes involving organic halides. Compounds containing h*2-bonded heteroalkene molecules including the C1 fragments CO2 and CS2, and related metallocyclic compounds are also emphasized in this volume. These types of compounds play an important role in a number of nickel-catalyzed co-oligomerization reactions involving unsaturated organic compounds. Most of the compounds are well-characterized by modern spectroscopic techniques. The various types of compounds are in most cases confirmed by X-ray diffraction. The author of these volumes, a member of the Mülheim group, has made many contributions of his own and is eminently qualified to critically review this area of research.