Highly electrophilic main group compounds: Ether and arene thallium and zinc complexes

This review describes aspects of the chemistry of highly electrophilic compounds of thallium and zinc. Extremely weakly coordinating anions based on perfluorophenyl borates allow the synthesis and structural characterization of complexes in which cation–anion interactions are minimized. The metals are therefore able to coordinate weakly donating ligands without the structural distortions and limitations in coordination geometry that are always encountered when coordinating anions are present. This has led to the isolation of thallium mono-, bis- and tris-arene complexes, thallium–ferrocene multidecker structures and the determination of their bond energies. New types of ether and crown ether adducts, as well as the first examples of cationic alkylzinc derivatives have become similarly accessible. These ionic Zn compounds very effectively polymerize cyclic esters and epoxides. Arene coordination is also a feature of strongly Lewis acidic bis(perfluoroaryl)zinc compounds, which when combined with activated alkyl chlorides provide an excellent new type of initiator for the homo and copolymerization of isoalkenes. These systems have given rise to a new family of elastomer materials.