The synthesis and characterization of hitherto hypothetical Au^III π‐alkyne complexes is reported. Bonding and stability depend strongly on the trans effect and steric factors. Bonding characteristics shed light on the reasons for the very different stabilities between the classical alkyne complexes of Pt^II and their drastically more reactive Au^III congeners. Lack of back‐bonding facilitates alkyne slippage, which is energetically less costly for gold than for platinum and explains the propensity of gold to facilitate C−C bond formation. Cycloaddition followed by aryl migration and reductive deprotonation is presented as a new reaction sequence in gold chemistry.

Catalytic activity in olefin polymerization depends not only on the catalyst but also, crucially, on activator/alkylator/scavenger “packages.” Along with binary mixtures containing Lewis or Brønsted acids and Al-alkyl systems, methylaluminoxane (MAO), a still ill-defined oligomeric compound, is the only single-component cocatalyst known to fulfill all three roles effectively and simultaneously. Herein, we report a simple molecular alternative, Al-H-Al⁺[B(C₆F₅)₄]⁻, an unusual borate salt containing a homodinuclear Al-cation (Al-H-Al⁺ = [iBu₂(DMA)Al]₂(μ-H)⁺). Unlike the simpler [AliBu₂]⁺[B(C₆F₅)₄]⁻, this species is easily synthesizable and stable at room temperature. Importantly, Al-H-Al⁺[B(C₆F₅)₄]⁻ can be used as a stand-alone cocatalyst for molecular olefin polymerization catalysis, representing an unprecedented molecular activator able to completely activate dichloride metallocene and prototypical post-metallocene precatalysts. Furthermore, spectroscopic and polymerization studies suggest that Al-H-Al⁺ is the true activating species formed in situ in the binary cocatalyst [PhMe₂NH]⁺[B(C₆F₅)₄]⁻/AliBu₃. As little as 50 equiv of Al-H-Al⁺[B(C₆F₅)₄]⁻ are required for efficient catalyst activation and impurity scavenging, orders of magnitude below the amounts usually required with MAO or AliBu₃. The high, yet “tamed,” Lewis acidity of cationic Al-H-Al⁺ is likely responsible for the increased scavenging ability. Unlike MAO, the well-defined structure of Al-H-Al⁺[B(C₆F₅)₄]⁻ offers easy avenues for further tuning, making it the prototype of a promising cocatalyst family.

The established model cluster (AlOMe)₁₆(AlMe₃)₆ for methylaluminoxane (MAO) cocatalyst has been studied by density functional theory, aiming to rationalize the different behaviors of unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethylaluminum; BHT = 2,6-di-tert-butyl-4-methylphenol), highlighted in previous experimental studies. The tendency of the three model Lewis acidic sites A–C to release neutral Al fragments (i.e., AlMe₂R; R = Me or bht) or transient aluminum cations (i.e., [AlMeR]⁺) has been investigated both in the absence and in the presence of neutral N-donors. Sites C are most likely responsible for the activation capabilities of TMA-rich MAO, but TMA depletion destabilizes them, possibly inducing structural rearrangements. The remaining sites A and B, albeit of lower Lewis acidity, should be still able to release cationic Al fragments when TMA-depleted modified MAOs are treated with N-donors (e.g. [AlMe(bht)]⁺ from MAO/BHT). These findings provide tentative interpretations for earlier observations of donor-dependent ionization tendencies of MAO and MAO/BHT and how TMA depleted MAOs can still be potent activators.