The paper summarizes new information on the formation and functioning of Ni(acac)2-based multicomponent (Ziegler) systems for oligo- and polymerization of ethylene. It turns out that catalytically active sites formed in these systems are deposited on nickel-containing nanoparticles generated via the interaction of the starting components directly in the reaction system. It is shown that the activity, productivity, and composition of the target products depend on the water content in the initial solvent, all other things being equal. A model of nanosized nickel-containing particles formed during the interaction of Ni(acac)2 and diethylaluminum chloride or ethylaluminum sesquichloride is presented.