Reactions at mild temperatures of the acetonitrile adducts [M(CO) 3 (NCMe) 3 ] (M ) Mo (1), W (2)) with various substituted cyclopentadienes in THF give the hydrido derivatives 17)) in isolated yields of 60-70%. THF solutions of derivatives 5a, 7a, and 8a undergo isomerization of the cyclopentadienyltethered-allyl unit to give the corresponding methyl-vinyl-dimethylsilyl-η 5 -cyclopentadienyl hydrido compounds) in final a:b ratios of ca. 2:1, 2:1, and 1:1, respectively. Solvent polarity seems to be crucial in these transformations. Dehydrogenation and desilylation of 5a in THF solutions to give the dinuclear derivative [Mo{η 5 -C 5 H 4 Si(CH 3 ) 2 (CH 2 CHdCH 2 )}(CO) 3 ] 2 ( 9) and [MoH(η 5 -C 5 H 5 )(CO) 3 ] (15), respectively, are processes competitive with the allyl isomerization. Treatment of 5a and 7a with equimolar amounts of ONMe 3 permits isolation of pure compound 9 and [W{η 5 -C 5 H 4 Si(CH 3 ) 2 (CH 2 CHdCH 2 )}-(CO) 3 ] 2 (10) in isolated yields of 80-90%. Coordination of the olefin pendant unit to the metal was detectable only by reaction of 5a and 7a with [CPh 3 ][B(C 6 F 5 ) 4 ] to give the cationic species [M{η 5 -C 5 H 4 Si-(CH 3 ) 2 (CH 2 CHdCH 2 )}(CO) 3 ][B(C 6 F 5 ) 4 ] (M ) Mo (18), W ( 19)). Molecular structure determinations by X-ray diffraction methods for derivatives 8a and 9 are also reported.

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