An unusual acetyl substitution of the cyclopentadienyl ligand is observed during the synthesis of allyl(η5-Cp)Mo(CO)2(I) be nucleophilic displacement on Cl(allyl)Mo(CO)2(NCMe)2with lithiocyclopentadiene according to Hayter. The origin of the acetyl group is established by deuterium labelling studies, and it is rationalized in terms of the activation of the coordinated acetonitrile to nucleophilic addition with LiCp. The X-ray crystal structure of (η5-AcCp)Mo(CO)2(η3-allyl) (II) reveals a slightly distorted cyclopentadienyl ring and an effective enlargement (i.e., expanded locus) of the ligand as a result of acetyl substitution. The stereoelectronic consequences of the substituted cyclopentadienyl ligand (η5-AcCp) are found in the relative populations of the exo- and endo-conformations of the coordinated allyl ligands in both II as well as in its derived cation IV (η5-AcCp)Mo(CO)(NO)(η3-allyl)+by comparison with their unsubstituted analogues I and III, respectively. The stereochemistry resulting from this steric change is also examined in the course of nucleophilic additions to the cationic allyl complexes IV with hydride, thiolate anions and carbanions. © 1986.
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