Low-coordinate rare-earth and actinide complexes

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Abstract As the chemical bonds formed between Rare-Earth (RE; Sc, Y, La, and Lanthanides Ln, Ce-Lu) or Actinide (An; Ac-Lr) ions and ligand atoms are predominantly electrostatic, they tend to form complexes where the coordination number (CN, formally the number of two center, two electron dative bonds between ligand atoms and RE/An ions) is maximized. The majority of RE and An ions are relatively large so CN > 6 tend to be observed in aqueous conditions, thus strict anaerobic conditions and sterically demanding ligands are required to achieve lower CN RE/An complexes. The coordinative and electronic unsaturation of the metal ion in low CN RE/An complexes can impart distinctive physicochemical properties (e.g., magnetism, small molecule activation, luminescence, unusual bonding regimes), which have inspired numerous studies in this area. The birth of low CN RE chemistry was in 1972, when Cotton et al. reported the structure of the first four-coordinate RE complex using aryl ligands, and Bradley and co-workers presented the synthesis of the first trigonal pyramidal RE complexes using bis(silyl)amides. Andersen reported the first trigonal pyramidal An complex in 1979, but it wasn't until Sattelberger's work in the late 1980s that structurally authenticated examples were provided. The fields of low CN RE and An chemistry are now blossoming, thus this chapter will cover all structurally characterized examples of formally two- and three-coordinate RE and An complexes to date, together with reactivity studies of these complexes. As well as providing a historical perspective, this chapter will cover ligand design criteria, the current state-of-the-art in this field and future directions to be exploited.