Establishing coordination numbers for the lanthanides in simple complexes

This article reviews the development of the understanding of the coordination number in lanthanide complexes, showing how it was realized in the 1960s that lanthanide complexes frequently had much higher coordination numbers than 6, and how it subsequently became possible for chemists to synthesise compounds with coordination numbers as low as 2, 3 and 4. Subsequent sections examine how coordination of solvent can cause uncertainties in coordination numbers; the determination of the coordination number of the aqua ions and in hydrated salts; variations (or not) in coordination number across the lanthanide series; the effect of counter-ion upon coordination number; and agostic interactions and interactions with distant atoms (when is a bond not a bond?). .