Kooperative Wirkung in π‐Ligand‐verbrückten Zweikernkomplexen, IX. Einfluß der Cp′XCp′‐Brücke in CpM(CO) 3 ‐Zweikernkomplexen (M  Mo, W) auf die Reaktion mit Alkinen 2

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 Me 2 Si[Cp′M(CO) 3 ) 2 ( 4 , Cp′ C 5 H 4 , M Mo, W) react thermally as well as photochemically with alkynes RCCR′ to form the monoalkyne compounds {Me 2 Si(Cp′M(CO) 2 ] 2 ‐μ‐(η 2:2 ‐RCCR′)} ( 6, 7, 9 ) which possess M 2 C 2 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′ NEt 2 ) a novel bis(carbene) complex 8 is formed, which contains a 1,2‐dimetallacyclobutadiene core in the central M 2 C 2 unit. In some cases products are isolated and characterized, in which three (R Ph, 10 ) and two (R n Pr, 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.