“Naked” Divalent Metal Cations Stabilized by Weakly Coordinating Anions: Syntheses, Properties, and Emerging Applications

The present contribution reviews the synthesis, structural characterization, and reactivity of “pseudo‐naked” M 2+ metal dications only stabilized by weakly coordinating anions (WCAs). The development of M[WCA] 2 salts that are soluble and stable in the condensed phase constitutes an emerging trend in metal‐based electrophilic species, which have traditionally relied, at least to some extent, on strong metal–ancillary ligand interactions for stabilization. For the most part, due to the high electrophilicity of the central M 2+ center, currently prepared M[WCA] 2 salts (M = Mg, Ca, Sr, Ba, and Zn) require the use of chemically robust WCAs such as halogenated carborates, [RCB 11 Cl 11 ] − , or alkoxyalanates of the type [Al{OC(CF 3 ) 3 } 4 ] − , with both classes of anions now being available through well‐established procedures. M[WCA] 2 salts are strong Lewis acids able to effectively catalyze a variety of chemical transformations under mild conditions in most cases, including the hydrosilylation of diverse unsaturated substrates (alkenes, alkynes, carbonyls, CO 2 ), imine hydrogenation, transfer hydrogenation of alkenes, carbonyl‐olefin metathesis (COM), and isobutene polymerization.