A Computational Approach to Determining Stability Constants for Transition Metal Ion Complexes

Jason St. Clair, Colby College

Document Type Honors Thesis (Open Access)


The complexation of metal ions in natural waters plays a critical role in defining the metal's biogeochemistry. Numerous investigators have used various experimental techniques to measure complexation constants in the laboratory, but many constants are still poorly undefined or have yet to be investigated due to the difficulty of some of the experiments. A computational approach to obtaining these constants for complexes of Fe (III), Fe (D), Cr (III), Cu (II), Zn (II), and Cd (II) was explored with varying degrees of success. Calculated Gibbs Free Energies were compared to experimental stability constants. Calculations were performed using Spartan's DFT package at the pEP level and with the DN* and DN** basis sets. Convergence success was in the following order, with the most successful first: Fe(ill), Zn(II), Cd(II), Cr(III), Cu(II), Fe(ll). Calculated hydrolysis constants for Fe(IU) and Zn(TI) followed a linear trend with experimental data. Fe(III) complexation with F, Cl-, and 6 CN- as ligands also formed a linear trend and had a slope (10.4) closer to RT (2.48) than that for Fe(l1I) hydrolysis (26.4). By accounting for the relative stabilization by solvation of the ligands, about half of this difference was closed.