Mole-Ratio Method for Determining Metal-Ligand Stoichiometry

An alternative to the method of continuous variations for determining the stoichiometry of metal-ligand complexes is the mole-ratio method in which the amount of one reactant, usually the moles of metal, is held constant, while the amount of the other reactant is varied. Absorbance is monitored at a wavelength where the metal–ligand complex absorbs. The illustrations below show typical results: (a) the mole-ratio plot for the formation of a 1:1 complex in which the absorbance is monitored at a wavelength where only the complex absorbs; (b) the mole-ratio plot for a 1:2 complex in which all three species—the metal, the ligand, and the complex—absorb at the selected wavelength; and (c) the mole-ratio plot for the step-wise formation of ML and ML2.


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