NEW SYNTHESIS OF MU-ALLENYLIDENE COMPLEXES FROM DIMOLYBDENUM CARBENIUM IONS CRYSTAL-STRUCTURE OF [(MO(ETA(5)-C5H5)(CO)(2))(2)(MU,ETA(2)-C=C=C6H10)]

A selective abstraction of the acetylenic proton of cationic allenyl c omplexes [{MoCp(CO)(2)}(2)(mu-eta(2),eta(3) HC=CCR(1)R(2))][BF4] (Cp=e ta(5)-C5H5) by using the acetylide LiC=CC(CH3)=CH2 gives rise to allen ylidene complexes [Cp(2)Mo(2)(CO)(4)(mu,eta(2)-C=C=CR(1)R(2))] ((CR(1) R(2)= [GRAPHICS] (1b); R(1) = R(2) = CH3, (2b); R(1) = H, R(2) = C2H5 (3b); R(1) = H, R(2) = CH3 (4b); R(1) = R(2) = H (5b); R(1) = CH3, R(2 ) = C6H5 (6c). Compounds 3b and 4b exist as two isomers while for 6c t he formation reaction is stereoselective. The crystal structure of [Cp (2)Mo(2)(CO)(4)(mu,eta(2)-C=C=C6H10)] (1b) has been determined.