Co-operative Effect in π-Ligand-Bridged Dinuclear Complexes, IX. - Influence of Cp′ - X - Cp′-Bridged Dinuclear CpM(CO)3 Complexes (M Mo. W) on Reactions with Alkynes The Cp′ X Cp′-bridged dinuclear complexes Me2Si[Cp′M(CO)3)2 (4, Cp′ C5H4, M Mo, W) react thermally as well as photochemically with alkynes RCCR′ to form the monoalkyne compounds {Me2Si(Cp′M(CO)2]2-μ-(η2:2-RCCR′)} (6, 7, 9) which possess M2C2 dimetallatetrahedrane units (X-ray structure analysis of 7a). A molecular fluxionality is shown for R R′ (7) by NMR spectroscopy. The activation barrier for this process depends on the steric bulkiness of the alkyne substituents R. For this kind of dynamic behaviour a new mechanism is proposed. In the reaction of 4 (M Mo) with the electron-rich alkyne bis(diethylamino)ethyne (R R′ NEt2) a novel bis(carbene) complex 8 is formed, which contains a 1,2-dimetallacyclobutadiene core in the central M2C2 unit. In some cases products are isolated and characterized, in which three (R Ph, 10) and two (R nPr, 11) alkyne units are co-ordinated under C - C linkage. Based on spectroscopic results a structural proposal is made for 10. By X-ray structure analysis of 11 it is shown that besides a dimerisation of the two alkynes an isomerisation occurs with formation of a complexed vinylallene.