Linear free energy relationships for metal–ligand complexation: Monodentate binding to negatively-charged oxygen donor atoms

The analysis presented below suggests that the following equation models monodentate binding to negatively-charged oxygen donor atoms with no distinction between phenolic, carboxylic or inorganic hydroxide functional groups: log KML = αO log KHL + βO; where KML is the metal–ligand formation constant, KHL is the corresponding proton–ligand formation constant, and αO and βO are termed the Irving–Rossotti slope and intercept, respectively. Linear free energy relationships (LFERs) of this type are presented for 24 different metal ions complexing to negatively-charged oxygen donor atoms. Ligands selected for use in LFERs meet the following criteria: (i) they contain negatively-charged oxygen donor atoms (e.g., carboxylic acids and phenols), (ii) they are capable of only monodentate binding to metal ions, (iii) steric hindrances are not expected to influence the extent of metal–ligand binding, and (iv) the negatively-charged oxygen donor atom is the only functional group that imparts charge. The intercept of all LFERs was nearly zero for all metal ions investigated (βO ≈ 0). The magnitude of αO indicates the relative preference of metal binding to negatively-charged oxygen donor atoms and to the proton. Values of αO can be used in QSARs (quantitative structure activity relationships) to estimate metal–NOM (natural organic matter) binding constants employed in the Windermere Humic Aqueous Model (WHAM) version V.