The properties of lanthanide (Ln) compounds have made them prime research candidates in the field of magnetism, 1 optics, 2 and catalysis. Control of magnetism and optical properties, which arise from the 4f electrons, is highly desired in materials science. The stereotypical understanding of the 4f electrons is often considered to be localized (core-like). They do not undergo large crystal fields (CF) splitting; thus, Ln(III) ions (except for Ce(III) or Ln(II) ions) in coordination complexes mainly behave like free ions.