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Bis(propynyl)zirconocene (1b) reacts with 0.5 molar equiv of trityl tetraphenylborate by a propynyl group transfer to form Ph3CC⋮CCH3 and Cp2ZrC⋮CCH3 +. The in-situ-generated (propynyl)zirconocene cation is not stable under the reaction conditions but instantaneously reacts with the neutral starting material 1b to form the [(μ-C⋮CCH3)(μ-CH3C⋮CC⋮CCH3)(ZrCp2)2 +][BPh4 -] salt, 9. Complex 9 was characterized by an X-ray crystal structure analysis. It contains an unsymmetrically bridging hexadiyne ligand and exhibits a planar-tetracoordinate carbon center that is stabilized by a three-center two-electron interaction with the two adjacent zirconocene moieties. In solution, complex 9 exhibits dynamic NMR spectra that indicate a symmetrizing exchange of the bonding situations between the two ends of the μ-hexadiyne ligand. The Gibbs activation energy of the automerization reaction of 9 is ΔG ⧧(190 K) ≈ 9.5 kcal mol-1.

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