The ruthenium hydride complexes C 5 Me 5 Ru(L)H 3 (L ) PPh 3 (1a), PCy 3 (1b), PMe 3 (1c)) were found to catalyze the dimerization reaction of terminal alkynes RCtCH (R ) Ph, t-Bu, SiMe 3 , CH 2 Ph, C 4 H 9 ) to produce cis-and trans-1,4-disubstituted enynes RCHdCHCtCR and 1,3-disubstituted enynes CH 2 dC(R)CtCR. The selective product formation was effected by modulating both the catalyst environment and the alkyne substrates. A rare form of dimer, cumulene PhCH 2 CHdCdCdCHCH 2 Ph (5d), was cleanly obtained from the dimerization of HCtCCH 2 Ph with 1b. A mechanistic interpretation is presented on the basis of the product distribution.

The in situ generated ruthenium acetylide complex C5Me5(PPh3)RuC⋮CPh (1) was found to catalyze the cross-coupling reaction of terminal and internal alkynes to yield functionalized enynes. An intermediate β-agostic species was isolated from the reaction of C5Me5(PPh3)(Cl)RuCCHBu-t (2b) with 2-butyne in the presence of NaOMe.

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