IntroductionThe last decade has evidenced an ever -increasing interest in the nanometric size chemical species area. More particularly, a great number of studies have been based on the synthesis and applications of noble transition metal colloids [1,2] . The development of soluble metal nanoparticles and colloids as highly active nanocatalysts has been the focus of considerable effort and several research groups have contributed signifi cantly to this central fi eld of nanosciences and nanotechnology [3 -5] . Due to their particular matter state, between homogeneous and heterogeneous, these frontier species are sometimes called " semi -heterogeneous " or " nanoheterogeneous " catalysts. From now on, soluble noble metal nanoparticles are considered as an unavoidable family of catalysts under mild conditions [6 -10] .Contrary to classical heterogeneous catalysts, metal nanocatalysts -generally defi ned as particles between 1 and 10 nm in size -can be obtained by the bottomup approach in two strategic ways according to the nature of the precursor, namely metal salts and organometallic compounds. Moreover, noble metal nanoparticles can be synthesized by a variety of methods according to the " organic " or " aqueous " nature of the media and the type of stabilizers used. The choice of the reaction parameters can provide some degree of control of particle size and particle composition, which is important for high activity and selectivity in catalytic applications. Finally, this fi eld has also allowed, from recent kinetic and mechanistic studies, noble transition metal colloids to be registered as effective catalysts.Over the past decade, nanoparticles, sometimes also called giant clusters, nanoclusters or colloids have been investigated as nanocatalysts in various catalytic applications such as hydrogenation, C − C coupling and other original reactions. This chapter reviews the recent progress in catalysis with ruthenium and rhodium nanoparticles as soluble nanocatalysts in various liquid media, organized according to the catalytic reaction type.
349Nanoparticles and Catalysis. Edited by Didier Astruc