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Treatment of the ij3-allyl complex Cp(CO)2Mo(CH2CHCH2) with 1 equiv of HBF4 generates the tj2-propene compound Cp-(CO)2Mo(CH2=CHCHj)(FBF3) (7), a covalent molecule containing ligated propene and FBF3~, as a rapidly equilibrating mixture (on the NMR time scale) of cis and trans isomers. Carbonylation (1 atm) of 7 thus gives the propene salt Cp(CO)3Mo(CH2= CHCH3)*BF4". Of more importance, 7 functions as a labile precursor to the formally 14-electron (or doubly unsaturated) Lewis acid Cp(CO)2Mo+. For example, both latent coordination sites on 7 are available for selectively forming the disubstituted derivatives Cp(CO)2Mo(PPh3)2+ and (Cp(CO)2MoI)2 upon treatment with PPh3 and , respectively, under mild conditions. The corresponding Lewis acid Cp(CO)3MoFBF3 (la) with only one latent coordination site (ligated FBFf), in contrast, gives only the monosubstituted derivatives Cp(CO)3Mo(PPh3)+ and Cp(CO)3MoI under comparable reaction conditions. A study was also instituted to see if the Lewis acids 7 and la convert Cp(CO)3MoCH3 and Cp(CO)3MoH to their bimetallic and symmetric M-(j;2-C,0)-acyl complexes [Cp(CO)2Mo]2(COR)+, where R = CH3 and H, respectively. [Note that using 7 instead of la now entails a net conservation of CO ligands in the formal Lewis acid promoted alkyl (or hydride) migration.] Reaction between 7 and Cp(CO)3MoCH3 indeed gives the known M-(7jz-C,0)-acetyl compound (42% yield vs. 36% from la), whereas 7 and Cp(CO)3MoH afford no discernible reaction products.

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