Nitrate and water in the first coordination shells of lanthanide complexes

During separations of rare-earth elements by solvent extraction, water and inorganic anions such as nitrate are often incorporated into the first coordination shells of the lanthanide (Ln) complexes with organic ligands (especially, the neutral ones). Understanding the coordination environments of Ln(III) ions, particularly the roles of nitrate and water in the first coordination shell, is critical for designing efficient and selective separation processes. Using the Cambridge Structural Database, we conducted a comprehensive analysis of crystal structures of Ln complexes, with a focus on the 29,891 mononuclear complexes and their first coordination shells, to extract and analyze structural features, including the presence, quantity, and coordination modes of nitrate and water ligands. We found that water and nitrate dominate the coordination environments that incorporate inorganic anions and/or solvent molecules, with distinct trends across the series. Nitrate most commonly adopts a bidentate coordination mode and contributes to higher average coordination numbers (from 10.15 for La to 8.84 for Lu) and more expanded first shells (average Ln-O_nitrate distance from 2.63 Å for La to 2.41 Å for Lu) compared to water (average coordination number from 9.07 for La to 8.21 for Lu; average Ln-O_water distance from 2.56 Å for La to 2.32 Å for Lu). Structures containing both nitrate and water exhibit coordination environments characteristic of nitrate-rich complexes, due to the popular occurrences of three nitrates and one water together in the first shells. This study provides a systematic view of nitrate and water in mononuclear Ln(III) complexes and offers insights that can inform the design of ligands for rare-earth separations.