Terminal, non-directional ionic “multiple” bond interactions between rare-earth metals and main group elements have posed a particular challenge in organolanthanide chemistry. Unlike for the 5f block elements, and more specifically for uranium, the rare-earth elements engage in predominantly ionic bonding, meaning that the high charge density of hard dianionic X2- functionalities, representing CR22-, NR2-, and O2– moieties, drives cluster formation with the relatively hard lanthanide cations. While terminal “unsupported” alkylidenes [Ln=CR2] have remained elusive, several terminal (organo)imides [Ln=NR] have been accomplished. On the other hand, the higher main group homologues are supposed to underly a Lewis acid base mismatch. Our attempts in this field aim at the synergistic interplay of bulky monoanionic ancillary ligands (kinetic stabilization) and new synthesis protocols, drawing on metal-methyl superbasicity. The multiple bond character of such terminal [Ln=X] is scrutinized by multinuclear NMR-spectroscopy and DFT calculations.
