Nucleophilic addition at u 3 -alkyne clusters leading to carbon-carbon bond formation

Reactions of nucleophiles with triosmium carbonyl clusters, especially those containing unsaturated hydrocarbon ligands, are discussed. Attack may be at CO, the metal atoms, at carbon of the organic ligand, or, where there are acidic metal-bound hydrogen atoms, deprotonation to give anionic clusters may occur. New results on the reactions of LiBHEt3 with p3-alkyne clusters of type Os3(CO)10 (RC2R') are considered in the light of the range of possible sites of attack. Protonation of anionic species that are formed gives hydrogenation products with or without the loss of CO. Os3H2(CO)9(RC2R') is usually a minor product, while C-C coupling leads to Os3H(CO)9(CRCR'COH) (in general the major product) and to Os3H(CO)9- (CRCR'CH). With terminal alkynes RC2H H-atom transfer accompanies C-C coupling to give Os3H(CO)9(RC—C =C H 2) in substantial amounts. The initial site of hydride attack (CO, alkyne or metal) is considered in the context of low-temperature 1H n.m.r. results.