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The heterobimetallic complexes [(η-C 5 H 5 )(OC)Ni-M(CO) 3 (η-C 5 H 4 Me)] (Ni-M, M ) Mo, W) react with 3-methoxy-3-methyl-1-butyne [HCtCMe 2 (OMe)] to afford the enantiomericwhich have chiral dimetalatetrahedrane cores, and the metallacycles [(η-C 5 H 5 )Ni{µ-η 3 (Ni),η 1 -formed via highly regioselective alkyne-CO coupling. All complexes react with HBF 4 ‚Et 2 O. The Ni-Mo µ-alkyne complex generated the propargylic cationic species [(η-C 5 H 5 )Ni(µ-HC 2 CMe 2 )-Mo(CO) 2 (η-C 5 H 4 Me)] + BF 4 -(Ni-Mo) by protonation, followed by rapid methanol elimination. Attempted protonation of the Ni-W µ-alkyne complex led to intractable products. However cationic Ni-W (µ-HC 2 CMe 2 ) complexes, and their Ni-Mo analogues, are accessible by direct protonation with HBF 4 ‚Et 2 O of the metallacycles [(η-C 5 H 5 )Ni{µ-η 3 (Ni),η 1 (M)-C(O)-C(R)-C(H)}M(CO) 2 (η-C 5 H 4 Me)] [Ni-M, M ) Mo, W; R ) CMe 2 (OMe)]. Proton migration and CO and MeOH elimination reactions rapidly follow to generate heterobimetallic µ-HC 2 CMe 2 cationic complexes. An intermediate was isolated from the slower Ni-W reaction. Spectroscopic data suggest that the stabilized HC 2 CMe 2 + ions are π-coordinated to the group 6 metal atoms and not to the nickel. Differences in dynamic behavior are observed between the Ni-Mo and Ni-W cations: rotation about the C-CMe 2 bond occurs at ambient temperature for the Ni-Mo allenyl complex on the 1 H NMR time scale, but the Ni-W allenyl species is static.

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