Estimating Stability Constants and Entropy for Reactions Between Aqueous Metal Ions and Monovalent Oxygen-Bearing Ligands: Applications to Hydrothermal Metal–Organic Acid Systems

Apar Prasad , Everett Shock

Linear free energy relations were obtained from existing experimental data and used to estimate stability constants for over 16,000 metal complexes with monovalent oxygen-bearing ligands comprising 75 metal species and 220 ligands. Similar relationships for metal-ligand entropy of association were also obtained which facilitate computation of stability constants from 0 to 125°C for over 6000 metal-ligand complexes. Our inventory for monovalent oxygen-bearing ligands includes carboxylic acids, phenols, alkyl alcohols, substituted alcohols and inorganic ligands with a special emphasis on geobiologically prevalent ligands like lactate, pyruvate, ascorbate and borate. Methods were also devised to estimate metal-ligand stability constants and complexation entropy if only the pKa of the ligand were known. The slopes and intercepts of these linear free energy relations can be explained by ligand denticity and metal ionic radius, as well as inductive and steric effects, thus providing chemical foundations for these estimation strategies. The stability constant estimates derived from this work were applied to model the speciation of metals and hexanoic acid in two hypothetical hydrothermal mixing fluids representative of the Rainbow and Lau vent fields. This enabled prediction of the distribution of biologically relevant chemical species across key state variables of the chemical system, including temperature, pH, and chemical composition when no stability constant information is available. We further explore the implications of these findings in the areas of metal transport in terrestrial and extra-terrestrial aqueous systems, deep-sea biology, origin and evolution of life, and environmental chemistry and biology.