Catalysts with P-, N-, or NHC-based ligands have been very important for studying the mechanism of Suzuki-Miyaura cross-coupling reactions. For example, complexes with ligands bearing NMR-active nuclei (i.e., F) have been used to monitor the reactions by NMR spectroscopy. Ligandless systems, i.e., those lacking strong donor ligands, have proved to be highly active catalysts when used at very low catalyst loadings (e.g., parts per million or billion), resulting in turnover numbers (TON) of up to 10 7 . Often, these palladium species are stabilized by a support (e.g., carbon, polymers, dendrimers, metal-organic frameworks (MOFs), quaternary ammonium salts (e.g., NBu4X, Jeffery system), or other additives (i.e., N,N-dimethylglycine). Identifying the composition and morphology of the catalytically active species in ligandless systems, which are generated from Pd salts or from Pd NPs, is a very challenging task. To a large extent, this is due to their low concentration and their dynamic character. Numerous studies indicate that a "cocktail" of Pd(0) species is involved, and when Pd NPs are used, these species are produced by Pd leaching from the NPs (Figure ). Despite intense research in this field, very little is known about the structure of the specific intermediates that may be involved in the catalytic cycle, more specifically, in the turnover-limiting step, which for aryl bromides and iodides is usually the transmetalation step. It is has been proposed that the oxidative addition step in the catalytic cycle may involve atomic Pd species in solution (Figure ). Palladium dimers have also been identified and isolated from the reaction mixtures, but these kinetically stable species were considered to be off-cycle intermediates that enter the catalytic cycle upon cleavage into monomers. Figure . Generally proposed catalytic cycle for the ligand-free Suzuki-Miyaura reaction and intermediacy of oxidative-addition palladium tetramers (this work). Some charges and/or counterions were omitted for clarity.