Limitations in Determining Stoichiometry During H2O to NH3 Exchange in Co2+ Solution

Abstract

Metal-ligand complexes are inorganic compounds with uses in both medicine and industry, as ligands often impart metals with different properties such as increased solubility. Metal-ligand exchange is the process by which ligands on metal centers can be swapped with other ligands. Because different colored metal-ligand complexes absorb characteristic wavelengths of light, UV-Vis spectroscopy can be used to monitor ligand substitution and estimate complex stoichiometry. This study examined the exchange system of hexaaquacobalt (II) ([Co(H2O)6]2+) to hexaamminecobalt (II) ([Co(NH3)6]2+) with the goals of stabilizing the reaction environment to allow the reaction to proceed without forming a Co(OH)2 precipitate, determining the complex stoichiometry using Job’s Method of Continuous Variations, and comparing these results with those obtained from the ligand exchange method. An organic buffer, Piperazine-N,N'-bis(2-ethanesulfonic acid) (PIPES), prevented precipitate formation, although the effect was not due to maintaining an acidic pH. Furthermore, it was discovered that absorbance values in mixed solution increased over time irrespective of additional ligand addition, likely due to the oxidation of Co2+ to Co3+. The oxidation change interfered with both Job’s method and ligand exchange analysis, limiting reliable determination of complex stoichiometry under the experimental conditions.