The cationic actinide complex [(Et2N)3U][BPh4] is an active catalytic precursor for the selective dimerization of terminalalkynes. The regioselectivity is mainly towards the geminal dimer but for bulky alkyne substituents, the unexpected cis-dimer isalso obtained. Mechanistic studies show that the first step in the catalytic cycle is the formation of the acetylide complex[(Et2N)2UC CR][BPh4] with the concomitant reversible elimination of Et2NH, followed by the formation of the alkyne p-complex[(Et2N)2UC CR(RC CH)][BPh4]. This latter complex (R tBu) has been characterized spectroscopically. The kinetic rate law is first order in organoactinide and exhibits a two domain behavior as a function of alkyne concentration. At low alkyne concentrations, the reaction follows an inverse order whereas at high alkyne concentrations, a zero order is observed. The turnover-limiting step is the C C bond insertion of the terminal alkyne into the actinide acetylide bond to give the corresponding alkenyl complex with DH? 15.6(3) kcal mol 1 and DS? 11.4(6) eu. The following step, protonolysis of the uranium carbon bond of the alkenyl intermediate by the terminal alkyne, is much faster but can be retarded by using CH3C CD, allowing the formation of trimers. The unexpected cis-isomer is presumably obtained by the isomerization of the trans-alkenyl intermediate via an envelope mechanism. A plausible mechanistic scenario is proposed for the oligomerization of terminal alkynes. The cationic complex [(Et2N)3U][BPh4] has been found to be also an efficient catalyst for the hydrosilylation of terminal alkynes. The chemoselectivity and regiospecificity of the reaction depend strongly on the nature of the alkyne, the solvent and the reaction temperature. The hydrosilylation reaction of the terminal alkynes with PhSiH3 at room temperature produced a myriad of products among which the cis- and trans-vinylsilanes, the alkene and the silylalkyne are the major components. At higher temperatures, besides the products obtained at room temperature, the double hydrosilylated alkene, in which the two silicon moieties are connected at the same carbon atom, is obtained. The catalytic hydrosilylation of (TMS)C CH and PhSiH3 with [(Et2N)3U][BPh4] was found to proceed only at higher temperatures. Mechanistically, the key intermediate seems to be the uranium–hydride complex [(Et2N)2U H][BPh4], as evidenced by the lack of the dehydrogenative coupling of silanes. A plausible mechanistic scenario is proposed for the hydrosilylation of terminal alkynes taking into account the formation of all products. ? 2000 Elsevier Science S.A. All rights reserved.