The chemistry of coordinatively unsaturated organoruthenium complexes is reviewed in this article. In particular, the subject is focused on neutral and cationic organoruthenium amidinates, which formally have 16 valence electrons and show signs of coordinative unsaturation. The ruthenium amidinates, (eta(5)-C5Me5)Ru(eta-amidinate) (1), and their isoelectronic analogues, [(eta(6)-arene)Ru(eta-amidinate)](+) (2), are synthesized and characterized; a possible stabilizing factor of the unsaturated metal center is weak coordination of pi-electrons in the amidinates ligands. Reactions of various two-electron donor ligands with 1 or 2 reveal the strong pi-donor property of 1 and Lewis acid nature of 2. One or two-electron redox processes of 1 in the reactions with organic halides are studied by isolation of the corresponding Ru(HI) and Ru(IV) products; the results lead to their catalysis for the Tsuji-Trost reaction and the intramolecular Kharasch reaction. The treatment of 2 with trimethylsilyldiazomethane results in the formation of cationic amidinato-carbene complexes, which involve unusual reversible metal-to-carbon silyl group migration. (C) 2003 Elsevier B.V. All rights reserved.